Add /pause command to toggle the runtime loop

[dgageot]

What

Adds a /pause slash command that pauses the agent runtime loop at iteration boundaries — i.e. as soon as the in-flight LLM request and any tool calls for the current turn complete. Running /pause again resumes the loop.

How

• pkg/runtime/pause.go (new): LocalRuntime.TogglePause() and waitIfPaused(ctx). The pause state is a single channel that is open while paused and closed on resume; closing broadcasts the wake-up to every blocked caller in O(1).
• pkg/runtime/loop.go: runStreamLoop calls r.waitIfPaused(ctx) at the top of its iteration loop. Respects context cancellation (returns ctx.Err() if cancelled while paused).
• pkg/app/app.go: App.TogglePause() returns (paused, supported bool); the second return is false for runtimes that don't expose the method (e.g. remote runtimes), letting the TUI render an explicit "not supported" message.
• TUI: New TogglePauseMsg, /pause command in the palette, and handleTogglePause showing a notification on each toggle ("Runtime paused — /pause again to resume" / "Runtime resumed" / "Pause is not supported with remote runtimes").

Tests

pkg/runtime/pause_test.go covers:

• toggle state cycles
• not-paused fast path
• block-until-resumed
• ctx cancellation while paused
• broadcast to multiple waiters (single resume wakes all)
• concurrent togglers + waiters under -race

All pass under go test -race -count=5 -run Pause ./pkg/runtime/. Full mise lint and mise test are green.

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Assisted-By: docker-agent

[docker-agent]

Assessment: 🟢 APPROVE

The pause/resume implementation is clean and correct. The channel-based broadcast design (close channel to wake all waiters in O(1)) is idiomatic Go. The mutex correctly guards the channel pointer without holding it during the blocking select, avoiding potential deadlocks. Context cancellation is properly handled. Test coverage is thorough, including race detection.