/talk has Stop: a turn can be ended mid-reply and its run's processes end with it #8818

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] While a reply streams, Send becomes Stop, and Escape stops too. Stopping ends the agent's run and every process it started, and keeps what was said so far, marked stopped.
• [x] Stop is in the API and the MCP.
• [x] A test starts a turn with the fake harness, stops it, and finds no process left and the partial turn kept.

Files: src/crew/conversation.ts, src/actions/talk.ts, src/web/pages/talk_client.ts, test/conversation-turn.test.ts Risks: a stopped run whose write was mid-confirm. A pending Confirm is cancelled, never applied.

Reviewed pull #8835 at its head: ticked While a reply streams, Send becomes Stop, and Escape stops too. Stopping ends the agent's run and every process it started, and keeps what was said so far, marked stopped.; Stop is in the API and the MCP.; A test starts a turn with the fake harness, stops it, and finds no process left and the partial turn kept..