config: Phase 5 routing + summarize-on-evict example (commit #5)

Phase 5 commit #5 (final) per docs/PHASE5.md §11. Documentation-only;
commented-out example showing:
  - routing.auto            (per-request auto-routing toggle)
  - routing.classes         (class → model mapping; reasoning = nil
                             by default per R-N2 cost-safety)
  - routing.fallback        (single-hop retry to cloud on transport fail)
  - routing.fallback_model  (default "cloud" if uncommented)
  - context.summarize_on_evict + summarizer_model + max_summary_chars
    (shown INSIDE the context = {...} block above)

All defaults OFF — Phase 5 is opt-in across the board. Existing configs
without `routing` or `context.summarize_on_evict` behave identically to
Phase 4.

Phase 5 implementation complete:
  #1 3e57824  router.classify_model + 31-case corpus
  #2 03497b5  context summarize_fn callback + summary block in to_messages
  #3 40ea0b4  repl routing + fallback + summarize_fn wiring + :route/:fallback
  #4 -        (bundled into #3 since meta cmds are trivial additions)
  #5 (this)   config example block

Phase 5 verify-partial:
  - router.classify_model: 31/31 case corpus passes
  - context summarize-on-evict: mock callback fires correctly (additive
    + compress paths), summary suppressed under Norris, :reset clears it
  - repl meta cmds: :route on/off/classes/check + :fallback on/off all
    work; :route check reports class + "routing currently disabled"
    suffix when auto is off (N1)

Verify-pending: end-to-end with real broker (route a code question, see
it land on deep; kill local backend, see fallback fire to cloud).

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

config.lua

@@ -146,4 +146,49 @@ return {
     --     -- (cloud may have variable cost per session).
     --     summarizer_model = "fast",
     -- },
+
+    -- Phase 5 (docs/PHASE5.md): multi-model routing + cloud fallback +
+    -- summarize-on-evict. OFF by default — auto-routing can spend money
+    -- silently on the cloud preset; require explicit opt-in.
+    --
+    -- routing = {
+    --     -- Enable auto-routing per request. When true, router.classify_model
+    --     -- inspects each prompt and may switch the model for THAT request
+    --     -- only (the :model selection is preserved across requests).
+    --     -- Default false. Toggle at runtime with :route on / :route off.
+    --     auto = true,
+    --
+    --     -- Class → model mapping. nil = "keep current" (heuristic fires
+    --     -- but no override). Ships with reasoning = nil because mapping
+    --     -- "explain ..." prompts to a paid cloud model would spend money
+    --     -- silently — opt in by uncommenting the reasoning line below.
+    --     classes = {
+    --         code      = "deep",      -- code-like prompts to local deep
+    --         -- reasoning = "cloud",  -- OPT-IN: "explain"/"why"/"how does" → paid
+    --         -- default   = nil,      -- keep active model
+    --     },
+    --
+    --     -- Single-hop retry on transport failure (HTTP 5xx, 408,
+    --     -- 404 model_not_found, DNS, connection refused, timeouts).
+    --     -- Retries against fallback_model once. Skipped if any text
+    --     -- has already streamed (no partial-output duplication).
+    --     -- Toggle at runtime with :fallback on / :fallback off.
+    --     fallback       = false,           -- default off (cost-safety)
+    --     fallback_model = "cloud",
+    -- },
+
+    -- ── Phase 5 context summarization on sliding-window eviction.
+    -- Set INSIDE the context = { ... } block above to enable:
+    --     context = {
+    --         max_turns          = 40,
+    --         token_budget       = 4096,
+    --         summarize_on_evict = true,
+    --         summarizer_model   = "fast",   -- model name in models{}
+    --         max_summary_chars  = 2000,
+    --     },
+    -- When summarize_on_evict is true, evicted turn pairs are fed to
+    -- summarizer_model and the result lives on ctx.summary, appended to
+    -- the system prompt as [earlier conversation summary]. Suppressed
+    -- in Norris mode (R-C4 — planner stays on its goal). If broker
+    -- fails, falls back to Phase 0 silent eviction (no crash).
 }