Add publishing — use changesets

Set up package publishing using [Changesets](https://github.com/changesets/changesets) (version bump + changelog + npm publish workflow driven by per-PR changeset files).

## Scope to nail down
- Where does Thunderjar get consumed from today — is it a published npm package, or only used via relative import (`../thunderjar/src/...`) as shown in the docs? Publishing implies a package boundary/public API needs to be settled first.
- Changesets bot/CI wiring — version PR workflow vs. direct publish on merge.
- Depends on [Add build pipeline] existing first, since publish should gate on CI passing.

Start by checking the documentation references to ../thunderjar/src/... and determine how Thunderjar is consumed today. Then review the existing build pipeline work before defining the package boundary and public API, Changesets bot/CI behavior, version-PR versus direct-publish flow, and CI gating; done means those choices are settled and the publishing workflow is specified.

Written by the indexing model from the issue text.