fix: Auto-bump to next canary after stable releases (#11761)
## Summary
- After a stable release (major/minor/patch), the release PR now
automatically bumps `version.txt` to the next prepatch canary before
merging to main.
- This prevents the hourly cron from continuing to produce canaries for
the already-released version (e.g. `2.8.3-canary.5` after `2.8.3` was
already shipped).
## How it works
The `create-release-pr` job reads the tag from line 2 of `version.txt`.
If it's `latest` (stable release), it runs `./scripts/version.js
prepatch` and commits the result to the staging branch before creating
the PR. When the PR squash-merges to main, `version.txt` lands with the
next canary baseline (e.g. `2.8.4-canary.0`), and the hourly cron
correctly produces `2.8.4-canary.1` on its next run.
For canary releases, the step is a no-op.
## Context
After `2.8.3` shipped, nobody manually ran `prepatch` to start the
`2.8.4` canary cycle. The cron kept producing `2.8.3-canary.X` releases
instead of `2.8.4-canary.X`.
