#20827 fix(cmux-next): enforce SSH, git, and capture deadlines (#20776)

Summary

A hung SSH process now stops when its deadline expires instead of leaving the caller waiting for the process to exit. Git writes terminate the entire child process group with a SIGTERM grace period and SIGKILL escalation, and failed screencapture exits are reported as failures instead of user cancellation.

The focused regression target covers all three paths with real subprocesses. Fixes #20776.

Testing

Added CmuxNextErrorHandlingTests with coverage for the SSH deadline, git child-process-group cleanup, and non-zero capture exit status.

The local checkout has no Swift/Xcode toolchain (swift test --package-path Packages/macOS/CmuxNext --filter CmuxNextErrorHandlingTests could not start: swift: command not found), so local execution is unverified. Hosted cmux-next checks are required for compilation and execution.

Changelog

Fixed: SSH, git, and screen-capture subprocess failures now honor deadlines and exit status.

Proof

No visual proof: this is subprocess error handling.

Checklist

• Behavior changes have added or updated tests, or Testing says why not
• User-facing docs updated if needed
• Reviewed with a subagent before merge (cmux-review), and all bot and human review comments resolved

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Summary by cubic

Fixes subprocess deadline and exit-status handling in cmux-next so hung child processes stop instead of leaving callers waiting, and failed subprocess exits report as failures. Fixes #20776.

• SSH processes terminate when their deadline expires; the cancellation reaches the inner await directly.
• Git timeouts signal the entire child process group with SIGTERM, escalating to SIGKILL after a two-second grace period; child processes start in their own group via posix_spawn, so hooks and filters cannot outlive a timed-out git.
• screencapture runs that exit nonzero are reported as failures with a localized error instead of success with cancelled: true.
• Adds CmuxNextErrorHandlingTests covering all three paths with real subprocesses, and registers it in test route selection and the CI target graph.
• Local execution is unverified because the Swift toolchain is unavailable.