fix: handle stdin EPIPE during codex exec

[pollychen-lab]

Summary

• reject through the normal runCodexExec() promise when writing the prompt to child stdin fails
• ignore later close and error events after the first failure settles the execution promise
• add a regression covering a fast fake Codex exit that would otherwise surface an unhandled EPIPE

Fixes #153.

Validation

• pnpm test -- --test-name-pattern 'stdin EPIPE|workspace-write as the default legacy sandbox'
• pnpm run check

[sylvesterkaczmarek]

The lifecycle handling looks sound here. Stdin errors now settle the same promise that owns the child run, and the settled guard stops a later child close/error from racing a second result. The large-prompt regression also makes the EPIPE path meaningfully reachable.