[ACP] Add terminal command timeout with kill_terminal_command

## Parent Epic

#926

## Description

`AcpShellExecutor` waits indefinitely for terminal exit. Add configurable timeout. On timeout, call `kill_terminal_command`, collect output, then release terminal.

## Acceptance Criteria

- [ ] Configurable timeout (default 120s)
- [ ] kill_terminal_command called on timeout
- [ ] Output collected after kill
- [ ] Exit code reflects killed state