fix: flush un-retained turns in SessionEnd hook before daemon stop

[octo-patch]

Fixes #1145

Problem

When retainEveryNTurns > 1 (e.g. the default of 10), the Stop hook skips retain on non-multiple turns. Turns between the last multiple and session end are silently dropped — a typical 27-turn session retains only up to turn 20, losing turns 21-27 completely.

Solution

session_end.py now calls _flush_remaining_turns() before stopping the daemon:

• Reads turn_count from state (without incrementing) and checks turn_count % retain_every_n != 0
• Skips when retainEveryNTurns == 1 — Stop hook already covers every turn
• Skips when turn_count % retainEveryNTurns == 0 — the last Stop hook already retained
• Uses session_id as document_id so the flush upserts the same document as prior full-session retains (last write wins, no duplicate storage)
• Adds flush_reason: session_end to metadata for observability
• Guarded by config.get("autoRetain") — no-op when auto-retain is disabled

Testing

Added TestSessionEndHook (4 tests) to test_hooks.py:

• test_flushes_remaining_turns_on_session_end — verifies flush fires when turn 1/3 was skipped
• test_skips_flush_when_retain_every_n_is_one — no extra retain when already retained every turn
• test_skips_flush_when_last_stop_already_retained — no double-retain after turn 3/3
• test_skips_flush_when_auto_retain_disabled — respects autoRetain: false

All 137 existing tests continue to pass.