feat: context re-injection with periodic and keyword triggers

Previously, supermemory context was only injected on the first message
of a session. This meant topic changes mid-session lost relevant
memory context.

Changes:
- Add reinjectEveryN config (default 0 = first only)
  Set to e.g. 10 to re-inject context every 10 messages
- Add recall keyword patterns (English + Japanese)
  'recall', '思い出して', 'メモリ確認' etc. trigger immediate
  context re-injection on that message
- Clean up session state on session.deleted
- Track per-session message counts instead of boolean set

Config example (supermemory.jsonc):
  { "reinjectEveryN": 10 }