ci: run stats on canary pushes for historical trend tracking (#88157)
## Summary
- Add push trigger for canary branch to `pull_request_stats.yml`
- Remove redundant `releaseStats` job from `build_and_deploy.yml`
- Rename workflow/jobs from "PR Stats" to "Stats"
## Background
The stats system was designed to save historical metrics to KV for trend
tracking, but it wasn't working:
1. `releaseStats` job only ran on actual stable releases (rare)
2. When it did run, KV secrets were missing
3. PR stats had KV secrets but didn't save (because `isRelease=false`)
This change consolidates stats into one workflow that runs on both PRs
and canary pushes.
## Behavior
**For PRs:**
- Compare PR branch vs canary HEAD
- Post comment to PR
- Don't save to KV
**For canary pushes:**
- Compare current canary vs last stable release tag
- Post comment to the commit
- Save stats to KV for historical trend tracking
## Test plan
- [ ] Verify workflow triggers on PR (existing behavior)
- [ ] Verify workflow triggers on canary push (new behavior)
- [ ] Verify stats are saved to KV on canary push
- [ ] Verify trend sparklines appear in PR comments (once history builds
up)