Add OSS maintenance readiness checks

[Yesol-Pilot]

What changed

• replace the unavailable PyPI install command with a commit-pinned GitHub install
• add Linux and Windows CI for Python 3.9, 3.12, and 3.13
• build and inspect wheel/sdist artifacts
• dogfood neo-evidence-gate on pull request descriptions
• add an evidence-first pull request template

Why

The README referenced pip install neo-evidence-gate, but the project is not published on PyPI. The repository also lacked public CI evidence. This PR makes the install path truthful and establishes reproducible maintenance checks before the first release.

Impact

Contributors get a working install command, a clear evidence section in new PRs, cross-platform regression checks, and package metadata validation.

Validation

• python -m pytest -q → 12 passed
• neo-evidence-gate examples/good.md → exit code 0
• neo-evidence-gate examples/bad.md → exit code 1 with 4 expected findings
• fresh virtual environment install from commit 21330b9 → CLI example exited 0
• local non-isolated wheel and sdist builds → both created
• python -m twine check dist/* → wheel and sdist both passed

A first isolated Windows build attempt was blocked by this machine's pip/platformdirs registry lookup. GitHub-hosted CI is the authoritative isolated check for this PR.