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)

package.json

@@ -10,18 +10,31 @@
   "bin": {
     "gitea-axi": "dist/main.js"
   },
+  "repository": {
+    "type": "git",
+    "url": "git+https://git.alexion.dev/alexion/gitea-axi.git"
+  },
+  "homepage": "https://git.alexion.dev/alexion/gitea-axi",
+  "bugs": {
+    "url": "https://git.alexion.dev/alexion/gitea-axi/issues"
+  },
+  "publishConfig": {
+    "access": "public",
+    "registry": "https://registry.npmjs.org/"
+  },
   "files": [
     "dist",
     "skills"
   ],
   "scripts": {
     "build": "tsc -p tsconfig.build.json",
-    "prepublishOnly": "npm run build",
+    "prepack": "npm run build",
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
-    "test:e2e": "vitest run --config vitest.e2e.config.ts"
+    "test:e2e": "vitest run --config vitest.e2e.config.ts",
+    "test:pack": "vitest run --config vitest.packaging.config.ts"
   },
   "dependencies": {
     "@toon-format/toon": "^2.3.0",