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>

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.