feat(recall): session-scoped re-injection cooldown (ADR 0023)

[dakl]

What

Stops the recall hook from re-injecting the same memory on every on-topic
prompt within a session. A memory injected via recall earlier in the session
(within 30 min) is suppressed, keyed off a new session_id carried on each
retrieval row. See ADR 0023.

Changes

• MemoryStore — additive session_id column + index;
  recentlyInjectedInSession, recallReinjectionCooldown; recordRetrieval
  gains an optional sessionID.
• Recall hook (engram CLI) — filters confident hits through the cooldown.
• engram-eval — replays Resources/sessions.json through the gate + the
  real cooldown and reports redundant re-injection rate (with/without) and
  first-touch coverage (RetrievalMetrics.SessionInjectionReport).
• Migration fix — the session_id index is created in
  addMissingRetrievalColumns, not createSchema, so existing DBs upgrade
  cleanly instead of failing on open with no such column: session_id.
  Regression test migratesPre0023RetrievalsTableMissingSessionID added.

Also in this PR — precision retune (gate 0.10 → 0.09)

Reviewing gate precision while building the session metric, the contextual
distance ceiling is tightened 0.10 → 0.09: on the eval that drops the
negative false-positive rate 13% → 0% and lifts injection precision
0.47 → 0.54 with unchanged gate recall (93%) — the lexical (≥2-token) leg
holds recall while the tighter distance leg sheds off-topic injections. Recall is
precision-first (the hook runs every prompt, so a false positive bloats context
repeatedly while a miss is recoverable), so 0.09 — the knee, where neg-FP hits 0
before recall gives — is the operating point.

• RecallGate.proposed maxDistance 0.10 → 0.09 (+ rationale); ADR 0021
  gets a dated addendum (decision text unchanged).
• engram-eval — finer calib-0.09/0.08 rows + calib-0.08-lex0 (shows
  recall collapsing without the lexical floor); --dump-scores for ROC/PR.
• scripts/plot_threshold.py — ROC + precision/recall-vs-threshold plot
  marking the shipped + legacy thresholds (regenerable; dumps gitignored).

Verification

• swift build (all products incl. engram-eval) clean; make test green (100
  tests, incl. cooldown, session-metric, and the migration regression test).

Notes

• Includes eval/runs/*.json A/B run records (per the repo's --record
  convention).
• The migration fix is the piece that was breaking app/CLI startup on existing
  stores; it's required for this feature to land safely.

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