feat: add search config surface and LLM query expansion

[BYK]

Phase 4 of search improvements (depends on #48)

Adds a configurable search section and optional LLM-based query expansion for the recall tool.

New config: search section

{
  "search": {
    "ftsWeights": { "title": 6.0, "content": 2.0, "category": 3.0 },
    "recallLimit": 10,
    "queryExpansion": false
  }
}

• ftsWeights — BM25 column weights for knowledge FTS5 search. Tune how much title/content/category matches matter relative to each other.
• recallLimit — Max results per source in the recall tool before RRF fusion (default 10, range 1-50).
• queryExpansion — When enabled, the recall tool uses the configured LLM to generate 2-3 alternative query phrasings before search, improving recall for ambiguous queries.

Query expansion (search.queryExpansion: true)

When enabled:

1. The configured LLM generates 2-3 alternative phrasings of the user's query
2. FTS5 searches run for each variant (original + expansions)
3. All results are fused via RRF across all query variants
4. Original query naturally gets higher weight (appears first in fusion)

Implementation:

• Uses the same worker session pattern as distillation/curation
• 3-second timeout — if the LLM is slow, falls back to original query only
• Errors caught silently (logged) — never blocks search
• Registered as lore-query-expand hidden agent

Config wiring

• ftsWeights flows through to all BM25 searches in ltm.ts via ftsWeights() function (reads from config().search.ftsWeights)
• recallLimit used as the per-source limit in the recall tool
• client + searchConfig passed through createRecallTool()

Test coverage

• 7 new config tests: defaults, partial overrides, range validation, optional section