Adds a core sessions_search agent tool backed by a SQLite FTS5 index that
lives next to the transcript rows in the per-agent database. Indexing is
transactional: user/assistant text is indexed inside the same write
transaction that persists the event, deletes drop index rows in the same
transaction, and branch rewinds mark the session dirty so the next search
rebuilds it from the same visible-path resolution sessions_history uses
(which also lazily backfills doctor-migrated databases). Search executes
gateway-side behind an additive sessions.search method with a bounded
session-key allowlist; visibility, sandbox restrictions, and snippet
redaction mirror sessions_history.
Closes#100978
