ci: milestone management workflow

[andreahlert]

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`)

[andreahlert]

@skrawcz @elijahbenizzy the repo doesn't have any milestones right now. if we start creating them per release (e.g. 0.43.0), a workflow could auto-assign on PR merge and that feeds directly into changelog generation. question is whether you want that level of tracking or if it's just overhead for the current team size

[hussain-s]

Please assign it to me.