fix: handle Codex stdin errors

[original4422]

Summary

Handle errors from the spawned Codex stdin stream through the action's existing execution promise.

Fixes #153.

Root cause

runCodexExec() supervised child startup and exit events, but wrote the prompt through child.stdin without an error listener. If Codex exited before consuming a buffered prompt, the resulting EPIPE became an uncaught EventEmitter error and crashed the helper.

Changes

• attach the existing promise rejection handler to child.stdin before writing the prompt;
• add an end-to-end fake-Codex regression that exits before reading an 8 MiB prompt file;
• verify the wrapper fails without Node's uncaught Unhandled 'error' event path;
• rebuild the checked-in dist/main.js bundle.

Validation

• Focused regression failed on unmodified main with uncaught Error: write EPIPE.
• Focused broken-pipe regression — passed.
• pnpm run check — passed.
• pnpm test — 145 tests: 142 passed, 3 platform-dependent tests skipped on macOS.
• git diff --check — passed.

Risk

Low. Normal stdin delivery and child exit handling are unchanged. The new listener only routes a previously unhandled stream error into the promise that already owns execution failures.

[github-actions]

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[original4422]

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