{"title": "feat(cli): add `releasekit doctor` command to validate project setup", "body": "### Problem\n\nThere's no quick way to tell whether a project is ready for a `releasekit` release. New users hit confusing errors like `git tag failed (exit 128)` or `gh: not authenticated` deep in the pipeline and have to guess what's wrong.\n\n### Proposal\n\nAdd a `releasekit doctor` subcommand that runs read-only checks and prints a clear pass/fail report. Suggested checks:\n\n- `git` is on PATH and version >= 2.30\n- `gh` is on PATH, authenticated, and the auth has `repo` scope on the current remote\n- `package.json` exists, is valid JSON, and has a `name` and `version` field\n- A `releasekit.config.{json,js,cjs,mjs}` is present (warn if missing \u2014 defaults still work)\n- Working tree is clean\n- The remote URL is reachable\n- The latest tag (if any) parses as semver\n\n### Acceptance criteria\n\n- `releasekit doctor` exits 0 if all checks pass, 1 if any check fails, 2 if warnings only\n- Each check prints one line with a status icon and a short message; full report at the end\n- Add `src/doctor.ts` with a `runDoctor()` function and unit-testable check functions\n- Add a `doctor` command in `src/cli.ts`\n- Add at least 6 unit tests covering pass/fail paths for each check\n- Update README with a \"Verify your setup\" section\n\n### Suggested files\n\n- `src/doctor.ts` (new)\n- `src/cli.ts` (wire up command)\n- `src/doctor.test.ts` (new)\n- `README.md` (docs)\n- `CHANGELOG.md` (entry under \"Unreleased\")\n\n### Test plan\n\n- `npx vitest run src/doctor.test.ts` \u2014 all green\n- `npx vitest run` \u2014 full suite still green (21 existing tests unaffected)\n- Manual: `node ./dist/cli.js doctor` in a clean checkout of releasekit \u2014 exits 0, all checks pass", "labels": ["enhancement", "good first issue"]}