#24267 Agent chat: no stray Copy at the top left of a sent prompt

Leo asked, about a lone Copy icon at the top left of his sent prompt in the agent chat:

why is there a copy button for the prompt i sent on teh top left?

top left??? just remove it

What changes

• A sent prompt (a user row drawn as the person's bubble) no longer gets the transcript row's floating Copy. That button was moved to the row's empty left corner on 10-20 so it would not cover the bubble. Under keyboard focus or a selected range it then showed alone at the far top left.
• Kept: the action row under the bubble (time, ✓✓, Copy, Pin). A selected range and Cmd-C still copy the prompt.
• Tool rows keep their floating Copy. Answers keep theirs in the turn footer. User rows drawn as a line (another agent's message, a task's result) keep theirs at the right.
• The CSS rule that put the prompt's floating Copy at the left is removed, since nothing uses it now.

Tests (red first, in their own commit d4eeeb03)

• New transcript-copy.test.tsx: "a sent prompt's row has exactly one Copy, the one under its bubble; tool and answer rows unchanged". It asserts one Copy button in the prompt row, inside .cv-user__actions, and no .acpmux-row__copy. The tool row keeps its own floating Copy, and the answer row has none.
• Updated to match:
  • render-tool-open.test.tsx: the prompt's one Copy is under its bubble.
  • composer-line-breaks.test.ts: it clicks the action row's Copy.
  • chief-context-copy.test.ts: comment only.
  • The Edited files gallery state waits for the prompt's action row.

Observed / Not run

• Not run: the webview tests could not run here, because the fleet steps for the red commit and the head sat in a saturated queue and never started. CI's webview jobs run them on this PR.
• Before/after: the gallery advisory run on this PR shows the prompt states at the merge base and at this head.

Takes back #22855: Leo asked to remove the prompt's floating Copy that #22855 moved to the top left ("top left??? just remove it"); its CSS rule has no remaining user.

Leo-ask: ask-6e5da66bad
Leo-ask: ask-cbf6b700cc

Changelog

Fixed: a sent prompt in the agent chat no longer shows a stray Copy button at its top left.

Agent session: Air-Blue

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