feat: add npm publish readiness (task 0020)

Complete the distribution metadata and publish flow for the unscoped
`gitea-axi` package:

- add `repository`, `homepage`, and `bugs` to package.json
- add `publishConfig` (public access, npmjs registry) so `npm publish`
  needs no extra flags
- replace `prepublishOnly` with `prepack: npm run build`, so both
  `npm pack` and `npm publish` rebuild `dist/` first
- document the single-command flow in PUBLISHING.md
- add a packaging smoke-test tier (`test:pack`) that packs the real
  tarball, installs it globally, and drives the installed binary
  (`--help`, dashboard header, and `setup` finding the bundled skill)

vitest.config.ts

@@ -5,7 +5,7 @@ export default defineConfig({
     // Unit + integration tiers: fast, no external dependencies. The end-to-end
     // tier under test/e2e needs a live Gitea instance and runs via `test:e2e`.
     include: ["test/**/*.test.ts"],
-    exclude: ["test/e2e/**"],
+    exclude: ["test/e2e/**", "test/packaging/**"],
     coverage: {
       provider: "v8",
       reporter: ["text", "html", "lcov"],