feat(prompt-hook): graph-derived gate tier + confidence-tiered injection + gate telemetry (#1136)

The keyword gate (#1126) can never know a repo's domain nouns. This adds
the graph-derived tier the design discussion converged on: symbol names
are split into prose segments at index time (name_segment_vocab, riding
the insertNode write path), and the hook verifies a prompt's plain words
against them — "the state machine des commandes" → OrderStateMachine, in
any language whose technical nouns are Latin script.

Confidence now decides HOW MUCH to inject, not just whether:
- HIGH (keyword, or index-verified code token): full explore injection,
  unchanged — the validated adoption lever.
- MEDIUM (segment matches only): a ~500-byte pointer naming the matching
  symbols; the AGENT writes the explore query. Never runs explore, so a
  fuzzy match can't inject 16KB of wrong-feature context.
- Silent otherwise, as before.

Precision is derived from the repo's own naming statistics plus measured
FP fixes: co-occurrence (≥2 words on one name) always qualifies; a single
word must be ≥5 chars, cluster across 2–25 names (singletons are prose
coincidence: "deploy to production" → matchesNonProductionDir), match a
multi-segment name, and not be an English function/filler word (the one
place a word list is honest: identifiers are English, so only English
prose collides). Every candidate is re-verified against nodes before
being surfaced — vocab rows are proposals, deletions leave orphans by
design, a full index rebuilds from scratch, and sync heals pre-upgrade
databases (batched + yielding; emptiness captured at sync ENTRY so the
sync's own writes can't mask the backfill).

Schema v7 migration is DDL-only (instant; none of the #1067 row-churn
hazards). Gate outcomes roll up as anonymous usage counters
(prompt-hook-gate-<outcome>, names only, never content) through the
existing telemetry pipeline — recall becomes measurable, and the counters
are the agreed kill-criterion data for ever revisiting a local classifier.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/design/telemetry.md

@@ -72,6 +72,13 @@ Event types:
   (e.g. `codegraph_explore`, `affected`), `count`, `error_count`, and for MCP:
   `client_name`/`client_version` from the `initialize` handshake (`src/mcp/session.ts`
   `case 'initialize'` — plumbing to add; currently unread).
+  The prompt hook additionally rolls up its gate DECISION as `cli_command`
+  counters named `prompt-hook-gate-<outcome>`, outcome ∈ `high-keyword` /
+  `high-token` / `medium-segment` / `nudge-projects` / `noop-shape` /
+  `noop-no-index` / `noop-unverified` — decision names only, never prompt
+  content. This is the gate's measured recall/precision funnel: a rising
+  `noop-*` share against the `high`/`medium` tiers is the signal that the
+  gate (keyword table or segment matching) is missing real questions.
 - **`uninstall`** — one per `uninstall`/`uninit` run (churn signal). Props: `targets`.
 
 Volume math: rollups mean monthly events ≈ active machines × active days × distinct