fix: Prevent fd double-close and handle EINTR in PTY I/O

[2witstudios]

Summary

• ManifestWatcher.swift: deinit was closing the file descriptor twice — once via the DispatchSource cancel handler and again explicitly with close(fileDescriptor). Fixed by only closing via one path depending on whether a source exists.
• pty_manager.rs: Both the PTY read loop and write loop treated EINTR (signal interruption) as a fatal error/EOF instead of retrying. A stray signal could terminate the output reader or fail a write.

Test plan

• All 93 pu-engine tests pass (unit + integration)
• Verify ManifestWatcher still detects file changes after atomic writes
• Verify PTY output streaming isn't interrupted by signals during long-running agents

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: b0c932af-dc95-439a-bd73-5e0d5b705fb1

📥 Commits

Reviewing files that changed from the base of the PR and between 55aa882 and 4b4be33.

📒 Files selected for processing (2)

• apps/purepoint-macos/purepoint-macos/Services/ManifestWatcher.swift
• crates/pu-engine/src/pty_manager.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch pu/fix-fd-bugs

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