fix: Windows update entrypoint locking and idempotent releases

[jrob5756]

Problem

Three issues with the v0.1.6 release:

1. Windows update fails with entrypoint error: uv tool install can't overwrite conductor.exe because it's locked by the running process. Root cause: Path.rename() fails silently on Windows when .exe.old already exists from a previous interrupted update (Windows rename() doesn't overwrite).

2. Release workflow fails: gh release create fails when the release already exists (e.g. created manually before CI runs).

3. Missing constraints file: The v0.1.6 release had no constraints.txt artifact because the workflow failed before uploading.

Changes

src/conductor/cli/update.py

• Extract _rename_windows_exes() helper that targets both shutil.which() result AND ~/.local/bin/conductor.exe (standard uv entrypoint location), deduplicating by resolved path
• Use Path.replace() instead of Path.rename() — replace() overwrites existing .old files from previous interrupted updates
• Handle partial success: when uv installs the package but fails to copy the entrypoint ("Failed to install entrypoint"), report success with a restart note instead of a scary error

.github/workflows/release.yml

• Check if release exists before creating; if so, gh release upload --clobber artifacts instead of failing

Release assets

• Uploaded constraints.txt and constraints.txt.sha256 to the existing v0.1.6 release

Tests

• Added test_windows_entrypoint_failure_reports_success — verifies partial success handling
• All existing Windows update tests pass unchanged (58 tests in file, 1801 total)