feat(model-router): wave 3-C — /route forget <name|all|projects>

Clears first-use decisions from the tracked routing.json through the
existing writer: a name is forgotten in every table (confirmed, changed
with the row restored to its recorded shipped phase under guards, project
exceptions pruned) and asked again; all and projects ask a confirmation in
the engine dialog (Cancel first) and hold the single-dialog slot; nothing
is written when there is nothing to forget; the answer names what was
restored and the frontmatter floor to realign when one was aligned; the
route tool has no forget path. Docs name the new writer. Kit suite 232 → 284.

Contract .claude/tasks/contracts/2026-10-11-model-router-w3c-forget-1457.md,
plan r3: 3 lenses + 1 confirmation, feater + 3 rounds (one real defect),
GATE 0 MET, verifier at the cap on coverage (user-accepted), security PASS.

lib/effort-shift.md

@@ -51,7 +51,8 @@ Builtin `/effort` is NOT a lever inside a run: rows and routes outrank it.
 - First use of a row asks once, with context (Later, Keep or Change: model,
   then effort, then Everywhere or this project only); the answer is kept in
   `mods/model-router/routing.json`. `/route pending` lists what is still
-  unconfirmed, `/route ask off|on` toggles the dialog.
+  unconfirmed, `/route ask off|on` toggles the dialog, `/route forget
+  <name|all|projects>` takes a decision back.
 
 Measure the split any time: `python3 ~/.claude/lib/effort-audit.py`
 (thinking/output/cache tokens per scope, model and effort).