feat(cli, config): Escalate Ctrl-C through a signal router

[JeanMertz]

Ctrl-C handling moves from a broadcast channel of ad-hoc signals to a SignalRouter that owns the root shutdown token and a LIFO stack of scoped interrupt handlers (RFD 045). The first Ctrl-C press now always reaches the topmost active handler: the streaming loop, the tool execution loop, or a new turn-level handler that covers the gaps between turn phases (persistence, thread building, response processing) and commits partial content before ending the turn. A press with no registered handler, or a second press within interrupt.escalation_cooldown_secs (default 2s), begins a graceful shutdown; any further press exits the process immediately with code
130. SIGTERM always requests a graceful shutdown and SIGQUIT always exits immediately, both bypassing the handler stack.

Cancelling an interrupt menu itself with Ctrl-C now escalates instead of silently falling back to "Stop": both the streaming and tool menus commit partial content or cancel running tools, then begin a graceful shutdown. During tool execution, a Ctrl-C is declined while an interactive tool prompt is active so the prompt keeps handling it, and propagates to the next handler down the stack otherwise. A Ctrl-C during the stream-error backoff wait now cuts the wait short and opens the interrupt menu immediately instead of after the full delay.

jp_task::TaskHandler drops its separate force-cancellation token now that SIGQUIT exits the process directly; background task draining watches the router's shutdown token instead.

Docs: RFD 045 is marked Implemented, and RFD 092 (predictable and responsive interrupt escalation) is added as a follow-up proposal covering the shutdown watchdog, a turn menu, a generic fallback menu, and Ctrl-C handling inside active prompts.