use a line channel per task

[Canop]

Removes the (theoretical, only with non default settings) risk of having one line leak from a killed command to the next one.

[Copilot]

Pull request overview

This PR updates the execution plumbing so each task execution uses its own output-line channel, preventing output from a killed command from being observed by the next command under certain (non-default) configurations.

Changes:

• Move the line output receiver from MissionExecutor to TaskExecutor, creating a fresh channel per task execution.
• Add a “keepalive” sender to prevent select! from busy-looping on a disconnected line channel after a task ends.
• Refactor child-process shutdown to wait() exactly once and avoid panicking on kill() errors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/tui/app.rs	Switches the UI event loop to receive command output from the per-task receiver.
src/exec/executor.rs	Creates a per-task line channel, keeps it connected until task replacement, and refines kill/wait behavior.

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