docs(repo): Realign README, docs, and About copy

Align public docs and maintainer guidance with the current 1,304-skill catalog state and v8.4.0 release copy.

Refresh README credits for newly imported external sources, update user-facing count references, and document the post-release docs realignment in the changelog and walkthrough.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "antigravity-awesome-skills",
   "version": "8.4.0",
-  "description": "1,273+ agentic skills for Claude Code, Gemini CLI, Cursor, Antigravity & more. Installer CLI.",
+  "description": "1,304+ agentic skills for Claude Code, Gemini CLI, Cursor, Antigravity & more. Installer CLI.",
   "license": "MIT",
   "scripts": {
     "validate": "node tools/scripts/run-python.js tools/scripts/validate_skills.py",