feat!: v3 — drop server.json handling, validate manifest.json instead

[mgoldsborough]

Summary

• Stop treating `server.json` as an authoring surface. The mpak registry composes its `ServerDetail` discovery shape from each bundle's `manifest.json` server-side, so the per-bundle `server.json` file (and this action's validate-and-upload step for it) is no longer useful.
• Replace the old "Validate & prepare server.json" step with a new "Validate manifest.json" step that checks the mcpb v0.4 required fields and, when present, validates the optional reverse-DNS name override at `manifest._meta["dev.mpak/registry"].name`.
• Cut as v3 so bundles upgrade explicitly.

Breaking changes

• Removed the `Validate & prepare server.json` step.
• Removed the `server-json-found` action output.
• The action no longer uploads `server.json` to the GitHub release. Bundles that still ship a `server.json` file in their repo are unaffected at build time but should drop it as part of upgrading to `@v3`.

Compatibility

• Bundles on `@v2` keep working — the registry already ignores `server.json` files in releases (the `ServerDetail` it serves is composed from `manifest.json`).
• New bundles or version bumps should adopt `@v3` at their own pace.

Migration for bundle authors (one-liner)

```bash
git rm server.json

Optional: add a branded reverse-DNS name override to manifest.json

"_meta": { "dev.mpak/registry": { "name": "com.example/your-server" } }

git add manifest.json
git commit -m "drop server.json (mcpb-pack@v3 composes registry metadata from manifest)"
```

Then bump the action ref in `.github/workflows/release.yml`:

```yaml

• uses: NimbleBrainInc/mcpb-pack@v3
  ```

Test plan

• Workflow YAML syntactically valid (no `server.json` step references remain in `action.yml`)
• README examples bumped to `@v3` (10 occurrences updated, 0 `@v2` remaining)
• Manifest validation step rejects: missing `manifest.json`, invalid JSON, missing required fields (`manifest_version`, `name`, `version`, `description`, `server`), malformed reverse-DNS override
• Cut `v3.0.0` tag after merge to fire the release workflow + update the `v3` major tag