fix(shell): bound Windows post-exit pipe draining

[Hona]

Issue for this PR

Windows shell commands such as bunx agent-browser can leave descendants holding stdout/stderr open after the parent exits, preventing shell completion.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a 200 ms, Windows-only drain window after parent exit. If inherited writers still prevent EOF, signal EOF on captured stdout/stderr so the existing output pump can finish and flush its file. Normal closure cancels the timer; POSIX is unchanged.

Nine added lines in the existing spawner. No new package, native code, dependencies, or test files.

This is a bounded cutoff, not a lossless-output guarantee: unread trailing output and later descendant writes can be excluded, and later writes can encounter a broken pipe. Saving captured output to a file does not recover bytes still in the pipes.

How did you verify your code works?

• bun typecheck in packages/util: passed.
• Prettier and git diff --check: passed.
• Ran external experiments on this Windows host with Bun 1.4.0 and Node 24.14.1. Across 144 idle-descendant cases, all foreground bytes reached the capture before the parent-exit event, including 16 MiB per channel through PowerShell -> bunx -> a harmless local fixture.
• Six deliberately delayed-output controls confirmed that descendants can write later; this is not a lossless guarantee for arbitrary programs.
• Exercised the actual spawner at 1 second and 200 ms. It retained the complete 2 MiB foreground body and both tails at 200 ms, completing about 207-210 ms after observed parent exit instead of 1006-1009 ms. Writes at 50/150 ms were captured; writes at 250/500 ms received EPIPE.
• Cached agent-browser@0.35.1 source prints command responses before CLI exit and uses null/private pipes for daemon/browser output. The actual browser was not executed because of the host workaround, so browser-specific runtime verification remains outstanding.
• No repository test files added; experiment scripts and results stay outside the repository.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR