context: proactive periodic summarization (closes #101)

Closes #101 (FR-A from the 2026-05-17 German strategy analysis,
small-model improvement strategy 5: "History-Zusammenfassung via
local").

Phase 5 summarize-on-evict only fires at budget pressure — exactly
when the local model is already suffering. Small models benefit
from tight context from turn 1, not "after eviction". This commit
adds CADENCE-triggered summarization that fires every N appends
regardless of budget, folding turns older than `summarize_keep_recent`
into ctx.summary via the existing Phase 5 summarize_fn closure.

context.lua additions:

- New ctx fields: summarize_every_n_turns, summarize_keep_recent
  (default 4), _turns_since_summarize (counter).
- Context:append bumps the counter on every store.
- Context:enforce_cadence — the new entry point. Returns the
  number of turns folded (0 on no-op). Guards:
    * disabled (cfg unset OR summarize_fn unset) -> 0
    * not yet due (_turns_since_summarize < N) -> 0
    * Norris-active (Phase 5 R-C4 parity — planner stays on goal) -> 0
    * #turns <= keep_recent (nothing to fold) -> 0
    * summarize_fn returns nil/empty -> 0 (defer to enforce_budget later)
  Orphan-tool guard: when the fold slice would end on an
  assistant-with-tool_calls, peel back the right edge until the
  next live turn isn't role=tool. Strict chat templates reject
  tool-without-assistant-anchor (#87 already encountered this).
- If ctx.summary grows past max_summary_chars after the fold,
  compress in a second pass (same shape as enforce_budget's
  Phase 5 logic).

repl.lua wiring:

- ctx_opts continues to copy all config.context keys; the new
  summarize_every_n_turns / summarize_keep_recent fields flow
  through automatically.
- make_summarize_fn is now wired when EITHER summarize_on_evict
  OR summarize_every_n_turns is set (same closure, different
  trigger — Phase 5's #51 #issue eviction path uses it on budget;
  #101 uses it on cadence).
- New status_cadence_fold helper: "[aish] proactively summarized N
  older turns".
- ask_ai's existing enforce_budget call site now first fires
  enforce_cadence, then enforce_budget. Cadence comes first so
  the token estimate enforce_budget sees is the tighter post-fold
  one — no spurious eviction of turns we just summarized.
- Norris path NOT wired: enforce_cadence is a no-op there via the
  norris_active guard (consistent with Phase 5 R-C4).

18 inline unit cases for enforce_cadence:
  - cfg disabled / no summarize_fn / below cadence -> 0
  - cadence met -> exact fold count (N - keep)
  - summary contains folded contents; first/last live turn IDs match
  - cadence counter resets; second fold fires after another N appends
  - Norris-active -> suppressed
  - orphan-tool guard: peels back when last folded = asst+tool_calls
  - summary compression triggers when over max_summary_chars

E2E verified on hossenfelder:8082, summarize_every_n_turns=4 /
summarize_keep_recent=2:
  5 user turns -> 2 cadence fires:
    [aish] proactively summarized 2 older turns
    [aish] proactively summarized 4 older turns
  :cost detail shows main=5 calls, summarize=2 calls (matches fires).
  Estimated ctx token count: 180 (vs ~1000 unsummarized).

Flag-off path: no status, identical to pre-#101 behavior.

Regression: 87/87 safety, 31/31 router_model, repl loads.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

context.lua

@@ -56,6 +56,17 @@ function M.new(opts)
         summarize_fn         = opts.summarize_fn,
         summary              = nil,   -- rolling summary string
         max_summary_chars    = opts.max_summary_chars or 2000,
+        -- #101: proactive periodic summarization (cadence-triggered,
+        -- in addition to Phase 5's eviction-triggered path). When
+        -- summarize_every_n_turns is set AND summarize_fn is wired,
+        -- enforce_cadence() folds turns older than the last
+        -- summarize_keep_recent into ctx.summary every N appends.
+        -- Goal: keep the wire prompt tight from the start so small
+        -- local models aren't fed near-budget context until eviction
+        -- forces a fold. nil = disabled (existing behavior).
+        summarize_every_n_turns = opts.summarize_every_n_turns,
+        summarize_keep_recent   = opts.summarize_keep_recent or 4,
+        _turns_since_summarize  = 0,
         -- Phase 6 (#issue Phase 6 §6): project file-tree block, set by
         -- repl.lua via :tree meta or the cfg.project.auto_tree startup
         -- hook. nil = no block injected. Cached scan opts (depth /
@@ -117,6 +128,8 @@ function Context:append(turn)
         assert(turn.content, "context:append requires content for role=" .. turn.role)
     end
     self.turns[#self.turns + 1] = stored
+    -- #101: bump cadence counter so enforce_cadence knows when to fire.
+    self._turns_since_summarize = (self._turns_since_summarize or 0) + 1
 end
 
 -- Buffer captured shell-exec output. Per §6 (post user-test fix), exec output
@@ -388,6 +401,66 @@ function Context:to_messages(opts)
     return msgs
 end
 
+-- #101: proactive periodic summarization. Fires every
+-- summarize_every_n_turns appends, folding turns older than the last
+-- summarize_keep_recent into ctx.summary via summarize_fn. Returns
+-- the number of turns folded (0 if disabled / not yet due / nothing
+-- to fold / Norris-mode / callback failed).
+--
+-- Norris suppression (Phase 5 R-C4 parity): the planner stays
+-- focused on its goal anchor — folding history mid-loop would
+-- change its perceived progress.
+--
+-- Orphan-tool guard: never fold an assistant-with-tool_calls turn
+-- without its matching role=tool turn(s). When the slice would end
+-- on such an assistant, peel back until it doesn't (the unfolded
+-- tail then becomes part of the live window — temporarily larger
+-- than summarize_keep_recent, but chat-template-legal).
+function Context:enforce_cadence()
+    if self.norris_active then return 0 end
+    if not self.summarize_fn then return 0 end
+    if not self.summarize_every_n_turns then return 0 end
+    if (self._turns_since_summarize or 0) < self.summarize_every_n_turns then
+        return 0
+    end
+    local keep = self.summarize_keep_recent or 4
+    local n = #self.turns
+    if n <= keep then return 0 end
+
+    local fold_count = n - keep
+    -- Orphan-tool guard: peel back from the right edge of the fold
+    -- slice while the last folded turn is assistant-with-tool_calls.
+    while fold_count > 0 do
+        local last = self.turns[fold_count]
+        if last and last.role == "assistant"
+           and last.tool_calls and #last.tool_calls > 0 then
+            fold_count = fold_count - 1
+        else
+            break
+        end
+    end
+    if fold_count == 0 then return 0 end
+
+    local pair = {}
+    for i = 1, fold_count do pair[i] = self.turns[i] end
+
+    local ok, new_summary = pcall(self.summarize_fn, self.summary, pair)
+    if not ok or type(new_summary) ~= "string" or new_summary == "" then
+        return 0  -- failure: leave turns; eviction will handle them later
+    end
+    self.summary = new_summary
+    if #self.summary > self.max_summary_chars then
+        local ok2, compressed = pcall(self.summarize_fn, self.summary, nil)
+        if ok2 and type(compressed) == "string" and compressed ~= "" then
+            self.summary = compressed
+        end
+    end
+
+    for _ = 1, fold_count do table.remove(self.turns, 1) end
+    self._turns_since_summarize = 0
+    return fold_count
+end
+
 -- Evict the oldest pair (user + assistant) while we exceed max_turns
 -- OR token_budget (Phase 8 pillar 5). Returns total turns evicted.
 -- Caller is responsible for rendering the §8 status line.

repl.lua

@@ -361,7 +361,11 @@ function M.run(config)
     if config.context then
         for k, v in pairs(config.context) do ctx_opts[k] = v end
     end
-    if config.context and config.context.summarize_on_evict then
+    -- #101: summarize_fn is also needed for cadence-triggered
+    -- summarization (Context:enforce_cadence). Wire it whenever
+    -- EITHER feature is enabled — same closure, different trigger.
+    if config.context and (config.context.summarize_on_evict
+                           or config.context.summarize_every_n_turns) then
         ctx_opts.summarize_fn = make_summarize_fn()
     end
     -- Phase 8 (docs/PHASE8.md): when cfg.tokenize.use_endpoint is true,
@@ -674,6 +678,13 @@ function M.run(config)
         end
     end
 
+    -- #101: status line for cadence-triggered fold.
+    local function status_cadence_fold(n)
+        if n and n > 0 then
+            renderer.status(("proactively summarized %d older turns"):format(n))
+        end
+    end
+
     -- ── Phase 5: fallback eligibility per PHASE5.md §5 ──────────────────
     -- All transport-failure patterns must match against the err string
     -- as broker.lua emits it (with "transport: " prefix). The matcher
@@ -1146,6 +1157,11 @@ function M.run(config)
             -- loop body re-runs broker.chat_stream with the now-extended ctx
         end
 
+        -- #101: proactively fold older turns into ctx.summary on
+        -- cadence (when cfg.context.summarize_every_n_turns is set).
+        -- BEFORE enforce_budget so it sees a tighter token estimate
+        -- and doesn't evict turns we just summarized.
+        status_cadence_fold(ctx:enforce_cadence())
         status_evictions(ctx:enforce_budget())
 
         -- CMD: extraction on the final pure-text response only.