ci: milestone management workflow

Part of #715

### Problem

There is no milestone tracking for releases. PRs merged to \`main\` are not tagged with which release they will ship in, making it hard to generate accurate changelogs or know what is included in a given release.

### Solution

Add a GitHub Actions workflow that auto-assigns milestones to merged PRs:

1. When a PR is merged to \`main\`, assign the current open milestone (e.g. \`0.43.0\`)
2. When a PR is merged to a release branch (\`vX.Y-test\`), assign the corresponding milestone
3. Create milestones for upcoming releases

### Acceptance criteria

- [ ] Workflow auto-assigns milestone on PR merge
- [ ] Works for both \`main\` and release branches
- [ ] Document milestone naming convention (\`X.Y.Z\`)