feat(core): honor note timestamps from frontmatter

[phernandez]

Why

Markdown notes can already declare created and modified, but Basic Memory previously ignored those semantic timestamps and exposed filesystem mtime instead. That made imported and hand-authored note history unstable across indexing, moves, and materialization.

This is a clean replacement for the timestamp foundation attempted in closed PR #1089.

What Changed

• Parse canonical created and modified ISO 8601 values once, after YAML normalization.
• Carry typed semantic timestamps through prepared writes, local and DB-first persistence, indexing, search, directory results, and materialization.
• Keep checksum, path, size, and mtime as physical file bookkeeping without overwriting note semantics.
• Remove ORM updated_at auto-update behavior and preserve semantic timestamps on moves and incidental writes.
• Document canonical timestamp behavior and filesystem fallbacks.

Implementation Details

• Date-only and naive values use the machine's local timezone; explicit offsets retain their represented instant.
• Missing or null fields fall back independently to file ctime/mtime, or one operation-local timestamp when stats are unavailable.
• Invalid canonical values fail with field-specific errors instead of silently falling back.
• Non-Markdown resources continue to use stat-derived timestamps.
• No database migration or request-schema change is required.

Testing

• Parser coverage for date-only, naive, offset-aware, missing/null, invalid, epoch-zero, and unavailable-stat cases.
• Prepared and accepted-write coverage for typed timestamp propagation and move preservation.
• Local indexing, batch reindexing, materialization, search filtering, and directory result coverage.
• just fast-check
• just doctor
• Full SQLite unit/integration suites.
• Full PostgreSQL unit suite plus complete sharded PostgreSQL integration suite (the monolithic integration recipe's 600-second guard was exceeded after 73% passed, so all tests were rerun in MCP and non-MCP shards).

Risks / Follow-ups

This PR intentionally excludes managed timestamp injection, aliases, and frontmatter rewriting. Those write-policy changes remain scoped to #684 after this foundation lands.

Closes #238

[phernandez]

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[phernandez]

Fixed the Windows-only epoch fallback failure in 73fa520. Stat timestamps are now constructed as UTC-aware datetimes before conversion to local time, avoiding the Windows CRT rejection when epoch zero plus a negative local offset crosses into 1969. The existing epoch-zero regression now uses a portable UTC instant. Verified uv run pytest tests/markdown/test_entity_parser.py -q (13 passed), just fast-check, and just doctor. Latest-head CI and BM Bossbot approval are pending.

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