docs: fix contributor doc links

[charles-openclaw]

Proposed changes

This fixes a few contributor-facing documentation links that are stale on a case-sensitive checkout:

• AUTHORS.md now links to CONTRIBUTING.md with the file's actual case.
• CONTRIBUTING.md now links back to AUTHORS.md with the file's actual case.
• The pull request template now links to the existing root CONTRIBUTING.md from .github/ and asks contributors to target master, which is the only branch currently published by the repository.

Fixes

No issue filed; this is a directly verified documentation/link cleanup.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• I am making a pull request against the master branch (left side). Also you should start your branch off our master.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Verification

• Ran a focused Python assertion that the three updated Markdown links resolve to existing files and the stale develop/../master/CONTRIBUTING.md wording is gone.
• Ran git diff --check.

No unit tests were run because this only updates contributor documentation and the PR template.