fix(local-env): use context manager in _update_cwd to prevent fd leak

[chinadbo]

Summary

• open(self._cwd_file).read().strip() in _update_cwd leaks the file descriptor because the handle is never explicitly closed
• Replace with a with statement context manager to ensure prompt cleanup after each read

Test plan

• test_update_cwd_uses_context_manager — verifies open() is used with __enter__/__exit__ (context manager)
• test_update_cwd_closes_file_handle — verifies no fd leak over 50 repeated calls
• Existing test_local_env_blocklist.py and test_local_tempdir.py still pass (21/21)

[alt-glitch]

Likely duplicate of #15552 — same root cause: bare open().read() fd leak in LocalEnvironment._update_cwd.

[teknium1]

Thanks for this contribution, @chinadbo! This is an automated hermes-sweeper review.

The fix you proposed has already landed on main via a separate commit that addresses the same root cause:

• Commit fe6c86623 — fix: close file descriptor in LocalEnvironment._update_cwd (merged 2026-04-29)
• tools/environments/local.py line 457 now uses with open(self._cwd_file) as f: — identical to what this PR proposes
• This commit explicitly fixes fix: close file descriptor in LocalEnvironment._update_cwd #15552, which @alt-glitch flagged as the duplicate root cause

Closing as implemented on main. The additional tests in this PR (test_local_env_file_handle.py) could still be worth salvaging as a separate contribution if desired.