Record an interruption note when Esc stops an agent run

Problem\n\nIn mecatui, pressing Esc stops an active agent run, but the transcript does not record that the run was interrupted by the user. This makes the visible conversation ambiguous: a later reader cannot distinguish an intentional stop from an incomplete or failed trajectory.\n\n## Desired behavior\n\nWhen Esc requests cancellation of an active agent run, add a concise transcript-visible note stating that the user stopped the run. The note should be ordered consistently with the cancellation/terminal outcome and should not claim that work was rolled back.\n\n## Scope to determine\n\n- Cover the mecatui Esc cancellation path without changing unrelated cancellation semantics.\n- Decide whether the note belongs in client-local transcript state, the session event stream, or durable session history/event log.\n- Avoid duplicate notes when cancellation originates elsewhere or when Esc is pressed repeatedly.\n- Preserve the distinction between user-requested cancellation, provider/transport failure, and normal completion.\n\n## Acceptance idea\n\nA TUI test drives an active run, sends Esc, and asserts that the rendered transcript includes one user-stop/interruption note alongside the run's terminal state.