[APIView] Epic: Version-Aware PR Lifecycle

## Overview

Version-aware PR lifecycle for APIView. Fixes baseline comparison bugs, enables PR-to-release promotion on merge (preserving comments and avoiding blob duplication), and unifies PR cleanup under the retention system.

## Work Items

- [ ] #15001 — WI-10: PR Version Backfill & Status Tracking
- [ ] #15002 — WI-11: Version-Aware PullRequestManager
- [ ] #15003 — WI-12: PR Promotion-on-Merge
- [ ] #15004 — WI-13: PR Cleanup via Retention

## Dependency Chain

```
WI-3 → WI-10 → WI-11 ──┐
├── WI-12
WI-8 ───────────────────┘
WI-5 + WI-10 → WI-13
```

WI-12 (promotion-on-merge) requires both WI-8 (comments are version-scoped) and WI-11 (PR creation is version-aware). WI-13 (PR cleanup) requires both the retention system (WI-5) and PR status tracking (WI-10).