feat: Phase 7D — confidence decay on unreferenced cold memories

Daily job multiplies confidence by 0.97 (~2-month half-life) for
active memories with reference_count=0 AND idle > 30 days. Below
0.3 → auto-supersede with audit. Reversible via reinforcement
(which already bumps confidence back up).

Rationale: stale memories currently rank equal to fresh ones in
retrieval. Without decay, the brain accumulates obsolete facts
that compete with fresh knowledge for context-pack slots. With
decay, memories earn their longevity via reference.

- decay_unreferenced_memories() in service.py (stdlib-only, no cron
  infra needed)
- POST /admin/memory/decay-run endpoint
- Nightly Step F4 in batch-extract.sh
- Exempt: reinforced (refcount > 0), graduated, superseded, invalid
- Audit row per supersession ("decayed below floor, no references"),
  actor="confidence-decay". Per-decay rows skipped (chatty, no
  human value — status change is the meaningful signal).
- Configurable via env: ATOCORE_DECAY_* (exposed through endpoint body)

Tests: +13 (basic decay, reinforcement protection, supersede at floor,
audit trail, graduated/superseded exemption, reinforcement reversibility,
threshold tuning, parameter validation, cross-run stacking).
401 → 414.

Next in Phase 7: 7C tag canonicalization (weekly), then 7B contradiction
detection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/dalidou/batch-extract.sh

@@ -166,6 +166,17 @@ curl -sSf -X POST "$ATOCORE_URL/admin/memory/extend-reinforced" \
     log "WARN: extend-reinforced failed (non-blocking)"
 }
 
+# Step F4: Confidence decay on unreferenced cold memories (Phase 7D)
+# Daily: memories with reference_count=0 AND idle > 30 days → confidence × 0.97.
+# Below 0.3 → auto-supersede with audit. Reversible via reinforcement.
+log "Step F4: confidence decay"
+curl -sSf -X POST "$ATOCORE_URL/admin/memory/decay-run" \
+    -H 'Content-Type: application/json' \
+    -d '{"idle_days_threshold": 30, "daily_decay_factor": 0.97, "supersede_confidence_floor": 0.30}' \
+    2>&1 | tail -5 || {
+    log "WARN: decay-run failed (non-blocking)"
+}
+
 # Step B3: Memory dedup scan (Phase 7A)
 # Nightly at 0.90 (tight — only near-duplicates). Sundays run a deeper
 # pass at 0.85 to catch semantically-similar-but-differently-worded memories.