Relax PR metadata governance checks

[QuantumMisaka]

This PR relaxes the agent governance PR-body metadata check and simplifies the default PR template for human contributors.

Checklist

• I have linked an issue or explained why none is needed.
• I have listed verification commands/results, or explained why checks were not run.
• I have described user-visible, INPUT/docs, or core-module impact if applicable.
• I have noted any governance exception needed for this PR.

Linked Issue

Fixes #7596.

Unit Tests and/or Case Tests for my changes

• Commands run: python3 -m unittest tools/03_code_analysis/test_agent_governance_check.py
• Result summary: 38 tests passed.
• Checks not run, with reason: full project build was not run because this only changes the Python
  governance checker, its unit tests, governance docs, and the PR template.

What's changed?

• Downgrade PR metadata completeness from a blocking error to a warning.
• Limit the metadata completeness check to the three core PR sections: linked issue, test evidence, and
  change summary.
• Simplify the PR template from the expanded governance checklist into a concise human-facing template
  with short governance notes.
• Update tests and governance documentation to match the new warning-level behavior.

Governance Notes

• INPUT/docs changes: no runtime INPUT parameters changed; governance documentation was updated for the
  severity change.
• Core module impact: none.
• Exceptions requested: none.