[instructions] Sync github-agentic-workflows.instructions.md with v0.27.0

[github-actions]

Instructions Update - Synchronized with v0.27.0

This PR updates the github-agentic-workflows.instructions.md file based on documentation changes since the last release (v0.27.0, released October 31, 2025).

Changes Made

• Added forks: field documentation to frontmatter schema section explaining the deny-by-default behavior
• Added "Fork Security for Pull Requests" section under Trigger Patterns with three clear examples:
  • Default behavior (same-repo only)
  • Allow all forks with ["*"]
  • Allow specific fork patterns with ["org/*", "user/repo"]
• Added "Fork Security" subsection to Security Considerations highlighting the secure default
• Updated pull_request trigger example to show forks field usage inline

Documentation Commits Reviewed

• 48f19e3 - Change default fork behavior for pull_request triggers to deny-by-default (Change default fork behavior for pull_request triggers to deny-by-default #2970)

This commit introduced a major security feature where pull_request workflows now block all forks by default and only allow PRs from the same repository. Previously, omitting the forks field allowed all forks to trigger workflows.

Key Feature Documentation

Fork Security (New in v0.27.0)

Pull request workflows now use deny-by-default fork behavior:

• Default: Same-repo PRs only (all forks blocked)
• Explicit opt-in required: Use forks: ["*"] for all forks or specific patterns like ["trusted-org/*"]
• Security improvement: Protects against untrusted fork execution by default
• Generated condition: Uses repository ID comparison for reliable fork detection

Validation

• Followed prompting best practices (imperative mood, minimal examples)
• Maintained technical tone and brevity
• Updated only necessary sections (Frontmatter Schema, Trigger Patterns, Security Considerations)
• Verified accuracy against current codebase and PR Change default fork behavior for pull_request triggers to deny-by-default #2970
• Removed no content (additive changes only)
• Used clear, concrete YAML syntax examples
• Placed documentation in logical sections for discoverability

Notes

This is a critical security feature that should be well-documented for AI agents writing workflows. The deny-by-default approach significantly reduces the attack surface for repositories using agentic workflows with pull_request triggers.

AI generated by Instructions Janitor