session-memory Phase 0: Tier1/Tier2 store (T03)

- session_memory/core/store.py: SQLite rows + blob-dir bodies, idempotent
  ingest on (session_uid,seq), Tier1/Tier2 usage accounting, evict_raw that
  drops raw but preserves the digest; watermark columns authoritative
- tests/test_store.py: ingest idempotency, accounting, eviction invariant

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workplans/AGENTIC-WP-0002-session-memory-phase0.md

@@ -58,7 +58,7 @@ Codex/Grok work in Phase 0 (designed for, not built).
 
 ```task
 id: AGENTIC-WP-0002-T03
-status: progress
+status: done
 priority: high
 state_hub_task_id: "2387258e-ba6d-4a41-919e-f2f4e0822110"
 ```
@@ -72,7 +72,7 @@ Tier 1 (rows + blobs) and Tier 2, used by retention.
 
 ```task
 id: AGENTIC-WP-0002-T04
-status: todo
+status: progress
 priority: medium
 state_hub_task_id: "017d8e90-633a-49f2-b342-8690938798cd"
 ```