Add Ctrl-X keybinding that sends session/cancel to abort an in-flight turn

[lsaether]

Problem

While a session/prompt is in flight (the input is disabled, placeholder reads agent is processing…), there's no way to abort the turn from the TUI short of Ctrl-C, which quits the whole app and tears down the WS connection. Killing the connection is sufficient on the wire — acp-mux sees the WS close, sweeps the active turn — but it's a coarse hammer: you lose any other session state, any queued log scrollback, and you can't immediately reattach without re-doing initialize / session/new.

ACP defines a dedicated cancel channel for this: session/cancel is a notification (client → agent, no response expected) carrying just { sessionId }. The agent is expected to abort whatever it was doing and complete the turn with stopReason: "cancelled". acp-tui doesn't send this today.

Proposed solution

Bind Ctrl-X (or whatever key feels right — Ctrl-D conflicts with EOF, Ctrl-C is already taken by quit, Esc is consumed by the permission prompt) at the app level. When pressed:

1. If there's no active turn, no-op (or log a hint "no turn in flight").
2. If there's an active turn, send cli.notify("session/cancel", {"sessionId": self.acp_session_id}) and log [bold yellow]cancel requested[/] in the log.
3. Leave the input disabled — the agent's eventual stopReason: "cancelled" response will re-enable it via the existing on_input_submitted finally block.

Optional: while we wait for the cancel to take effect (some agents are slow to honor it), update the placeholder to cancelling… so the user knows the keystroke registered.

Implementation notes

• ACPClient.notify(method, params) already exists in src/acp_tui/acp_client.py:101-107. No new client code needed.
• The binding goes on ACPApp.BINDINGS with priority=True so it works even when PromptInput has focus.
• An action method action_cancel_turn is the handler. It needs to check self._prompt_in_flight (or whatever tracks "input disabled because a prompt is running") — currently the disabled state is the only signal. Either add an explicit flag on ACPApp or read prompt_input.disabled and assume the only thing that disables it is a turn-in-flight. The flag is cleaner.

Scope: ~20 lines plus a paragraph in the README.

Out of scope

• Sending session/cancel while a permission prompt is pending — that's a different mode (the prompt is the in-flight thing, not the agent's turn), and the existing [esc] cancel already covers it.
• Cancel-during-turn that interrupts a tool call mid-execution. ACP delegates the actual cancellation semantics to the agent; the TUI's job is just to send the notification.