CI: strengthen PR checks across OS/Python matrix & document local verification

[Joshua-Ward1]

Summary

• Update CI to run on push and pull_request targeting main

• Expand test matrix to Ubuntu, macOS, Windows × Python 3.11 / 3.12

• Disable fail-fast to surface all matrix results

• Enable pip caching and standardize installs using python -m pip

• Add CI steps for:

  • Ruff formatting check
  • Ruff linting
  • CLI smoke commands
  • pytest -q

• Ignore generated sysforge-report.json artifacts

• Update README to reflect CI-aligned local workflows

Verification

The following commands mirror CI behavior:

python -m ruff format --check .
python -m ruff check .
python -m pytest -q
sysforge --version