feat(workspace): copy final agent replies as markdown

[wen2zhou]

Related issue

None.

Problem

Agent replies render Markdown, but the existing copy action is available only for user prompts. Copying a final agent response should preserve its original Markdown source.

Proposed change

• Add a copy control below the final successfully completed agent reply for each user turn.
• Copy the original message content and show confirmation only after the clipboard write succeeds.
• Omit the control for intermediate, streaming, failed, and empty agent messages.

Scope and non-goals

• This change copies message Markdown text only.
• It does not add transcript export or include generated artifacts and images in the copied value.

Acceptance criteria and validation

• Final completed agent replies expose a left-aligned Markdown copy control.
• The clipboard receives the original Markdown, and failed writes do not show a success state.
• npm run typecheck passed.
• npm run lint passed with pre-existing repository warnings.
• Focused workspace tests passed: 40/40.
• npm run test is currently blocked by existing Compute integration failures: the test database is missing ComputeHost.executionBackend.

Review focus

• Final-response selection across intermediate agent messages and legacy sessions without response linkage.
• Clipboard success and failure state handling.