[Enhancement]: Add macOS/Windows runners to CI and catalogue POSIX assumptions

[shauryagangrade]

Problem

CI only runs on ubuntu-latest (the tests matrix varies Python versions,
not OSes). macOS and Windows are untested — and the code has known POSIX
assumptions (execute_bash with shell=True/bash, git tooling, path
handling).

Proposed approach

• Extend the tests matrix with windows-latest and macos-latest.
• Triage failures: mark clearly-supported expectations, skip what's
  explicitly Windows-unsupported, and file/close follow-ups.
• Add a docs/ note on the OS support matrix.

Where to look

• .github/workflows/ci.yml:40-68 (tests job/matrix)
• gcode/tools.py bash/git tools

Acceptance criteria

• CI runs the test suite on macOS and Windows.
• Every skip/failure on Windows is justified and referenced to an issue.

Difficulty

Medium — needs tolerance for CI iteration.