/talk says what each turn cost and how much context is left #8822

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] Under each reply, a quiet line gives the turn's cost in dollars. The conversation's header shows how full the context is, as a share, and suggests a new conversation past 80 %.
• [x] The figures come from the run's own usage record, not an estimate.
• [x] A test with a recorded usage event.

Files: src/crew/conversation.ts, src/web/pages/talk.ts, test/talk-page.test.ts

Reviewed pull #8846 at its head: ticked Under each reply, a quiet line gives the turn's cost in dollars. The conversation's header shows how full the context is, as a share, and suggests a new conversation past 80 %.; The figures come from the run's own usage record, not an estimate.; A test with a recorded usage event..