sessions: search session transcripts to find past discussions

Problem

When I remember a topic but not the session where it was discussed, there is no way to locate that conversation from its transcript. Session inventory and replay metadata help identify sessions, but require already knowing which session to open.

Proposed capability

Add transcript-based session search across persisted sessions. Given a query, return matching sessions with enough context to select and resume/replay the right conversation.

Acceptance criteria

• Search covers persisted session conversation/transcript content, including user and assistant messages.
• Results identify the session and include a concise matching snippet/context.
• Results can be narrowed by the existing session metadata where practical (for example workspace, date, or session type).
• The search surface is available to the applicable clients/API and respects the session store's retention and access boundaries.
• Define predictable behavior and documentation for empty queries, no matches, and stores/backends that cannot support efficient full-text search.

Notes

This complements #471 (session inventory and replay metadata) and #721 (pin/archive chats): inventory lets users browse known sessions, while transcript search lets them recover a session when they only remember the subject discussed.

Start by inspecting the existing session inventory and replay metadata work referenced in #471, then trace the session store and the applicable API/client entry points. Define how transcript content, metadata filters, access boundaries, unsupported backends, empty queries, and no matches should behave. Done means persisted user and assistant messages can be searched and results identify sessions with concise context, with documentation and tests covering the stated behavior.