Document PAT creation, secret configuration, fork PR restrictions,
and troubleshooting. Link from ci.md and repository.md.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds a Repository Security Policies section to SECURITY.md pointing to
the new branch protection and repository settings docs, fixes Matrix and
phone contacts to proper hyperlinks, and cross-links the security policy
files from pull-request.md, branch.md, and workflow.md. Applies
documentation.md rules to all touched files (RFC 2119 usage, emoji
placement, link text).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>