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>

src/types.ts

@@ -410,6 +410,25 @@ export interface SearchResult {
   highlights?: string[];
 }
 
+/**
+ * A symbol whose name-segments match prose words from a prompt — the
+ * graph-derived signal behind the front-load hook's medium tier
+ * (CodeGraph.getSegmentMatches). Always verified to exist in `nodes` at the
+ * time it is returned.
+ */
+export interface SegmentMatch {
+  /** Symbol name as indexed (e.g. `OrderStateMachine`). */
+  name: string;
+  /** Kind of the representative definition. */
+  kind: NodeKind;
+  /** File of the representative definition. */
+  filePath: string;
+  /** 1-based start line of the representative definition. */
+  startLine: number;
+  /** The prompt words (normalized) that matched this name's segments. */
+  matchedWords: string[];
+}
+
 // =============================================================================
 // Context Types
 // =============================================================================