feat(inbox-steward): add automated PR processing workflow

Automates stewarding PRs through CICD gate:
- Identifies PRs that passed all checks (Dogfood Gate, Scorecard, Hypatia)
- Validates against merge criteria
- Auto-merges qualifying PRs from trusted contributors
- Dispatches success events to .git-private-farm
- Reports to Hypatia for ruleset updates

Part of closed-loop automation with:
- gitbot-fleet: execution and dispatch
- .git-private-farm: cross-repo coordination
- hypatia: ruleset and monitoring

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Author: hyperpolymath