/talk searches across a person's conversations #8823

Follows #8752 (pull #8755 shipped copy, who and when, stick-to-bottom and the jump button, tool cards with Confirm and Cancel). Keep src/web/pages/talk.ts's server-drawn history and streaming, a small script, no React, and #8719's quiet rules.

Done looks like:

• [x] A search box on the conversation list finds conversations by title and by words in their turns, using the forge's existing search, never a table scan per keystroke.
• [x] A result opens the conversation at the matching turn.
• [x] Tests: a word in an old turn finds its conversation; another person's conversations never appear.

Files: src/core/conversations.ts, src/search/, src/web/pages/talk.ts, test/conversations.test.ts

Parked by the crew: the check stayed red through 2 automatic tries (pull #9484 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Reviewed pull #9691 at its head: ticked A search box on the conversation list finds conversations by title and by words in their turns, using the forge's existing search, never a table scan per keystroke.; A result opens the conversation at the matching turn.; Tests: a word in an old turn finds its conversation; another person's conversations never appear..