Make temporary_cd() thread-safe by not calling os.chdir() in the no-argument form

[Copilot]

os.chdir() is process-wide: concurrent threads calling temporary_cd() simultaneously all mutate the same CWD, causing races where threads write/read files in each other's temp directories.

Changes

• temporary_cd() (no-arg): no longer calls os.chdir(). Creates a temp dir, yields its absolute path, and cleans up — the process CWD is never touched. Thread-safe.
• temporary_cd(path) (explicit path): retains os.chdir() for backward compatibility. Still not thread-safe (unchanged behaviour, now documented).
• Return type: Generator[None, None, None] → Generator[str, None, None] — the yielded absolute path is the intended interface for file I/O.
• Absolute path is resolved before os.chdir() to avoid resolving a relative path against the new directory.

Migration

# Before — not thread-safe
with temporary_cd():
    open("f.txt", "w").write(data)

# After — thread-safe
with temporary_cd() as tmpdir:
    open(os.path.join(tmpdir, "f.txt"), "w").write(data)

New tests cover: correct yielded paths in all three call forms, and concurrent correctness with 8 threads.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (7bc4a2f) to head (7287329).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #147   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files           6        6           
  Lines         158      158           
=======================================
  Hits          150      150           
  Misses          8        8           

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