fix(fileops): remove temp file when atomic_write_file fails

[plusky]

atomic_write_file creates a mkstemp temp file, writes to it, then moves it
into place:

fd, fname = mkstemp(dir=path.parent)
with os.fdopen(fd, "w") as f:
    f.write(data)
move(fname, path)

If the write or the move raises (e.g. a permission or I/O error on the
destination), the temp file is left behind in the destination directory. Not
incorrect output, but litter accumulates on the error path.

Fix

Wrap the write + move in try/except; on any failure unlink the temp file
before re-raising (on a successful move it is already gone).

Test

test_atomic_write_cleans_temp_on_move_failure patches move to raise and
asserts the directory listing is unchanged (no temp leaked) and no partial
target exists. Verified it fails on the old code and passes on the fix.

Gates: ruff format/check clean, ty clean, pytest green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.44%. Comparing base (af6ceb2) to head (5708fda).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #215   +/-   ##
=======================================
  Coverage   86.43%   86.44%           
=======================================
  Files         159      159           
  Lines        8832     8838    +6     
=======================================
+ Hits         7634     7640    +6     
  Misses       1198     1198           

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