feat(search): add since/before date window to mempalace_search and CLI search (#463)

[mvalentsev]

What does this PR do?

Fixes #463

Adds an optional [since, before) date window to both search surfaces, filtered on each drawer's filed_at (the created_at shown in results):

• MCP: mempalace_search accepts since and before (ISO date or datetime; since inclusive, before exclusive).
• CLI: mempalace search gains --since / --before.

Semantics are shared with the list_drawers filter from #1128 through a new side-effect-free mempalace/date_window.py module (the #1128 helpers moved there; mcp_server keeps them under the old private names, so the existing list_drawers behavior and tests are untouched). Comparison is wall-clock and timezone-naive, Z/offset inputs are normalized, and drawers with a missing or unparseable filed_at are excluded while a bound is active, so a date-filtered result never silently includes rows of unknown age.

Design notes:

• ChromaDB rejects string operands for $gte/$lt (verified on the locked 1.5.7), so the window cannot be a server-side where clause. It is applied as a post-filter on every candidate path: the vector scored loop, candidate_strategy="union" lexical merge, and the BM25-only sqlite path used when vector search is disabled.
• A post-filter starves a fixed pool, so under an active window the vector candidate pool widens from n_results * 3 to max(min(n_results * 15, 500), n_results), the same ceiling the degraded-filter path already uses. When the widened pool still comes back full, the response carries date_filter_pool_truncated: true instead of passing a thin result off as complete; the BM25 path sets the same flag when its candidate page hits max_candidates.
• On the BM25 sqlite path the window is also pushed into SQL as a narrowing prefilter at whole-day granularity, which keeps it provably wider than the window for every ISO spelling that shares the date prefix (bare date, space separator, minute precision, Z/offset suffixes); the Python check stays authoritative, mirroring how wing/room are double-checked there.
• Parameter names are since/before rather than the issue's after: list_drawers already shipped since/before with these exact semantics in feat: add since/before date filter to list_drawers and search #1128, and one vocabulary across the read tools seemed more valuable than a second name for the same bound. Relative forms (--last 7d) and a --between shorthand from the issue discussion are intentionally left for a follow-up; they layer cleanly on top of this window.
• Known edges, called out rather than hidden: when ChromaDB fails a filtered query and search falls back to the unfiltered-plus-post-filter recovery path (fix: filtered search fallback + diary_write content alias #1245), total_before_filter reflects the post-filtered count, so the truncation flag can under-report there; union mode's lexical fetch keeps its n_results * 3 size (widening it under a window is a possible follow-up, the merge itself already respects the window); a before at the calendar ceiling (9999-12-31) skips the SQL narrowing instead of overflowing past datetime.max; and a stored filed_at with leading whitespace would pass the Python filter but not the SQL prefilter (no in-tree writer produces one).

How to test

Automated (all green locally, ruff 0.15.14 clean):

uv run pytest tests/test_date_window.py tests/test_searcher.py tests/test_mcp_server.py -q

New coverage: 20 unit tests for the shared parser/window, 26 searcher tests (window/boundaries/invalid/inverted, undated and aware filed_at, pool widening recall, truncation flags on both paths, legacy filed_at spellings and the calendar-ceiling sentinel on the sqlite path, union side door, CLI ordering and errors), 6 MCP-surface tests (schema, threading, filters echo, clean error shapes).

Manual, against a real palace:

mempalace search "budget meeting" --since 2026-04-01 --before 2026-05-01
mempalace search "budget meeting" --since garbage   # clear error, exit 1

MCP: call mempalace_search with {"query": "budget meeting", "since": "2026-04-01"} over JSON-RPC; the tool schema now declares both properties, results stay within the window, filters echoes it, and an invalid or inverted window returns a structured error instead of -32000/-32602.

Checklist

• Tests pass (python -m pytest tests/ -v)
• No hardcoded paths
• Linter passes (ruff check .)