session-memory Phase 0: budget-based retention sweep (T05)

- session_memory/core/retention.py: RetentionConfig + sweep() with backstop,
  budget (oldest-analyzed-first, never touches un-analyzed), and hard-cap
  overflow (analyze-now then reported last-resort data_loss); EvictionReport
- tests/test_retention.py covers all four branches

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

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

@@ -86,7 +86,7 @@ Signal extraction beyond the digest stays stubbed for the Detect phase.
 
 ```task
 id: AGENTIC-WP-0002-T05
-status: progress
+status: done
 priority: high
 state_hub_task_id: "89177c79-528e-4023-a7eb-67f8e0276ba9"
 ```
@@ -103,7 +103,7 @@ synthetic sessions and tiny caps.
 
 ```task
 id: AGENTIC-WP-0002-T06
-status: todo
+status: progress
 priority: medium
 state_hub_task_id: "a4b35c76-154d-4e99-b6d0-61cb6e47ecc0"
 ```