test(runs): make approval synchronization deterministic

[solomonneas]

Summary

• Replace wall-clock sleeps in the watch approval test with explicit thread gates.
• Keep the time override local to runs_cmd instead of mutating the shared standard-library module.
• Capture worker exceptions and re-raise them when the test joins the thread.
• Remove source-regex assertions in favor of deterministic behavioral coverage.

Verification

• Focused synchronization suite after merging current main: pass
• tests/repeat_watch_approval_test.py after merging current main: 20 runs passed
• Independent cross-file review: no blocking findings

Closes #664

[solomonneas]

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