Panic in AbsolutePathBuf::parent() under EMFILE (Too many open files)

Summary

When file descriptors are exhausted (EMFILE / "Too many open files"), Codex can panic in AbsolutePathBuf::parent().

Observed panic:

• parent of AbsolutePathBuf must be absolute: Os { code: 24, ... "Too many open files" }
• Followed by: in-flight tool future failed during drain: ... JoinError::Panic(...)

Why this happens

AbsolutePathBuf::parent() currently does this:

• calls self.0.parent()
• then re-validates via Self::from_absolute_path(p).expect(...)

from_absolute_path() uses path_absolutize::absolutize(), and path-absolutize calls std::env::current_dir() even for already-absolute paths.

Under EMFILE, current_dir() fails, so the expect(...) in parent() panics.

Impact

• An otherwise recoverable resource-exhaustion condition is escalated into a process panic.
• This is easiest to hit during high parallelism (many sub-agents/tool tasks).

Repro notes

This was observed with low soft ulimit -n (256) and heavy parallel tool/sub-agent activity.

Proposed fix

In AbsolutePathBuf::parent(), avoid re-absolutizing and avoid expect:

• construct directly from known parent path: Self(p.to_path_buf())

This preserves the invariant (parent of an absolute path is absolute) and removes the unnecessary current_dir() dependency on the hot path.

I have an isolated PR ready with this change plus tests.