feat(session): add anti-drift session notes

[vicary]

Summary

• add Redis-backed session notes with MCP read/write tools, note-aware session search, and shared runtime wiring
• inject full pinned note bodies into compaction-only session memory while keeping normal chat turns unchanged
• add plan/spec docs plus test coverage for note lifecycle, compaction gating, search bias, and root-session migration

Test Plan

• deno test -A
• deno task check
• deno task lint
• deno task fmt --check