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 0), graduated, superseded, invalid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audit trail, graduated/superseded exemption, reinforcement reversibility,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>

src/atocore/api/routes.py

@@ -1517,6 +1517,39 @@ def api_graduation_status() -> dict:
     return out
 
 
+class DecayRunBody(BaseModel):
+    idle_days_threshold: int = 30
+    daily_decay_factor: float = 0.97
+    supersede_confidence_floor: float = 0.30
+
+
+@router.post("/admin/memory/decay-run")
+def api_decay_run(body: DecayRunBody | None = None) -> dict:
+    """Phase 7D — confidence decay on unreferenced memories.
+
+    One-shot run (daily cron or on-demand). For active memories with
+    reference_count=0 and idle for >30 days: multiply confidence by
+    0.97 (~2-month half-life). Below 0.3 → auto-supersede with audit.
+
+    Reversible: reinforcement bumps confidence back up. Non-destructive:
+    superseded memories stay queryable with status filter.
+    """
+    from atocore.memory.service import decay_unreferenced_memories
+
+    b = body or DecayRunBody()
+    result = decay_unreferenced_memories(
+        idle_days_threshold=b.idle_days_threshold,
+        daily_decay_factor=b.daily_decay_factor,
+        supersede_confidence_floor=b.supersede_confidence_floor,
+    )
+    return {
+        "decayed_count": len(result["decayed"]),
+        "superseded_count": len(result["superseded"]),
+        "decayed": result["decayed"][:20],  # cap payload
+        "superseded": result["superseded"][:20],
+    }
+
+
 @router.post("/admin/memory/extend-reinforced")
 def api_extend_reinforced() -> dict:
     """Phase 6 C.3 — batch transient-to-durable extension.