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/db/migrations.ts

@@ -9,7 +9,7 @@ import { SqliteDatabase } from './sqlite-adapter';
 /**
  * Current schema version
  */
-export const CURRENT_SCHEMA_VERSION = 6;
+export const CURRENT_SCHEMA_VERSION = 7;
 
 /**
  * Migration definition
@@ -100,6 +100,25 @@ const migrations: Migration[] = [
       `);
     },
   },
+  {
+    version: 7,
+    description:
+      'Add name_segment_vocab — prose-word → symbol-name lookup for the prompt hook’s graph-derived gate',
+    up: (db) => {
+      // DDL only — instant on any size database (the row-churn hazards of #1067
+      // don't apply). The table starts EMPTY on migrated databases; `sync`
+      // detects that over a populated graph and backfills batched+yielding
+      // (CodeGraph.rebuildNameSegmentVocab), and any full index rebuilds it
+      // from scratch. Keep the definition in lockstep with schema.sql.
+      db.exec(`
+        CREATE TABLE IF NOT EXISTS name_segment_vocab (
+          segment TEXT NOT NULL,
+          name TEXT NOT NULL,
+          PRIMARY KEY (segment, name)
+        ) WITHOUT ROWID;
+      `);
+    },
+  },
 ];
 
 /**