session-memory Phase 0: session digest + outcome heuristic (T04)

- session_memory/core/digest.py: build_digest (cost totals, kind/tool
  histograms, markers, snippets) + cross-flavor infer_outcome heuristic;
  analyze() promotes Tier1->Tier2 and sets analyzed_at (-> evictable)
- tests/test_digest.py

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

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

@@ -72,7 +72,7 @@ Tier 1 (rows + blobs) and Tier 2, used by retention.
 
 ```task
 id: AGENTIC-WP-0002-T04
-status: progress
+status: done
 priority: medium
 state_hub_task_id: "017d8e90-633a-49f2-b342-8690938798cd"
 ```
@@ -86,7 +86,7 @@ Signal extraction beyond the digest stays stubbed for the Detect phase.
 
 ```task
 id: AGENTIC-WP-0002-T05
-status: todo
+status: progress
 priority: high
 state_hub_task_id: "89177c79-528e-4023-a7eb-67f8e0276ba9"
 ```