Signal handling in Omni/Agent: SIGINT cancel, SIGTERM graceful exit

Goal

Make Omni/Agent respond to standard Unix signals the way interactive programs do:

• SIGINT (Ctrl-C) → cancel the current iteration. The Op loop check should raise SteeringCancel (or equivalent), unwinding the current turn cleanly. In stdin-mode, after cancellation the agent returns to reading the next prompt. In one-shot mode it exits with a cancellation status.
• SIGTERM → finish the current iteration and then exit cleanly (no more prompts accepted). In one-shot mode it behaves the same as today.
• SIGHUP → ignore, or optionally treat like SIGTERM. TBD by the implementer.

Why signals instead of JSON commands

Pi uses {"type":"cancel"} on stdin. We're not doing that — cancel is what SIGINT was designed for, it's what every REPL, shell, and interactive Unix program uses, and it doesn't require inventing a command vocabulary. Keeps stdin as a pure prompt channel.

Acceptance criteria

• kill -INT <pid> during an in-flight loop iteration causes the current LLM call or tool to unwind cleanly via the existing SteeringCancel path in Op.check.
• In stdin-mode, after SIGINT the agent returns to reading the next null-delimited prompt (does not exit).
• kill -TERM <pid> causes the agent to finish the current iteration (don't cancel mid-turn), then exit 0 without reading another prompt.
• Signal handlers are installed only when running in stdin-mode (or always — implementer's call, document the choice).
• Existing agent script.md shebang case is unaffected except that SIGINT now behaves like a cancel instead of a hard kill (which is a strict improvement).

Depends on

t-759.1 (stdin prompt mode must exist before the "return to read next prompt after cancel" behavior makes sense).

Implementation notes

• Use System.Posix.Signals (installHandler, sigINT, sigTERM).
• Probably set an IORef or TVar flag that the Op loop's existing check step polls.
• There's already a SteeringCancel plumbing in Op.check — reuse it, don't invent new cancellation machinery.