Stop terminal activity after interrupted turns (#2942)

* fix(terminal): clear activity when turns are interrupted

Some terminal agents do not emit a stop lifecycle hook after a user interrupt. Treat exact interrupt key frames as cancellation at the terminal input boundary so running state cannot stick, and clear rather than finish to avoid completion attention.

* test(terminal): use platform shell in interruption tests

Let the terminal test fixture select the platform default shell so the interruption cases reach their assertions on Windows.

docs/terminal-activity.md

@@ -53,6 +53,11 @@ Claude hook mapping:
 - `Stop`, `StopFailure`, `SessionEnd` → `idle`
 - `Notification` with `reason` or `matcher` equal to `idle_prompt` → `needs-input`
 
+Claude does not run `Stop` when the user interrupts a turn. A standalone Ctrl-C or Escape input
+while terminal activity is working clears the activity without finished attention. The same
+fallback applies to every hooked terminal agent; exact input matching excludes escape sequences and
+pasted content, while later provider idle events remain authoritative.
+
 Codex hook mapping:
 
 - `UserPromptSubmit` → `running`