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.packaging.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    // The packaging tier: pack the real tarball, install it globally into a
+    // throwaway prefix, then drive the installed binary. It builds, packs, and
+    // fetches runtime deps from the registry, so it is far slower than the fast
+    // tiers and runs on its own via `test:pack`. Distribution touches no Gitea
+    // API, so this smoke test is the distribution analogue of the live-Gitea
+    // e2e tier rather than a member of it.
+    include: ["test/packaging/**/*.test.ts"],
+    testTimeout: 180_000,
+    hookTimeout: 300_000,
+    // Keep the pack/install work in one process: the tarball is built once in a
+    // shared setup and reused across the assertions.
+    fileParallelism: false,
+    passWithNoTests: true,
+  },
+});