Sync github-agentic-workflows.md with v0.46.1 documentation (#16675)

- Remove custom engine (removed in v0.46.1)
- Remove copilot-sdk engine reference (removed in v0.46.1)
- Add runtimes.*.if conditional field for setup steps
- Add assign-to-user blocked/unassign-first fields
- Add unassign-from-user allowed/blocked fields
- Add create-issue group/close-older-issues fields
- Update engine enum and breaking changes section
- Correct roles field to on.roles with deprecation note

Co-authored-by: gh-aw agent <gh-aw-agent@github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people