perf(session-search): skip unused context enrichment via fields projection (salvage #63389)

[kshitijk4poor]

Salvage of #63389 by @frizikk — both commits cherry-picked verbatim (authorship preserved). No follow-up fixes needed: the verification pass came back clean.

What this does for users

Every session-search (the dashboard's search box AND the agent's session_search discovery tool) ran one extra SQL query PER MATCH to build a "surrounding context" window — which both consumers then threw away (the router copies only metadata/snippet fields; the tool hydrates its own windows via get_anchored_view). With the router over-fetching max(limit*5, 50) matches and the tool scanning up to 300, that's up to hundreds of wasted queries per search.

This adds an optional fields= projection to search_messages(): the context-enrichment loop is skipped when the projection doesn't include context, and both discard-callers opt in. Omitting fields preserves the complete legacy result — no behavior change for any other caller (verified: the router and the tool are the only two production callers).

Measured impact

Synthetic 50 sessions × 200 messages, FTS5 on, 15 reps, median:

Case	FULL	PROJECTED	Δ	context queries
~20 matches, limit=100	1.44 ms	0.29 ms	−79.6%	20 → 0
~100 matches, limit=100	7.20 ms	1.18 ms	−83.6%	100 → 0
~100 matches, limit=20	1.69 ms	0.48 ms	−71.4%	20 → 0

Honest caveat: absolute numbers are from a small synthetic DB (~0.06ms/context query); the win scales with DB size and match count, but a single search is not a hot loop — this is per-user-search latency, not per-turn.

Verification

• 204 passed (base 201 + 3 new); trivial auto-merge cherry-pick
• Mutation check: forcing context_matches = matches (defeating the skip) fails test_search_projection_skips_context_enrichment_queries — the test counts context queries via sqlite trace callback, so it binds the mechanism
• Adversarial: context loop mutates nothing but match["context"]; both field tuples validated member-by-member against _SEARCH_MESSAGE_RESULT_FIELDS (a typo would 500 every dashboard search — none present); every field each consumer reads ⊆ its projection (id included in the tool's, excluded from the router's, matching actual usage)
• Attribution chore chore: add frizikk to AUTHOR_MAP #77636 merged (frizikk → AUTHOR_MAP)

Closes #63389.