chore(docs): refresh README token stats and bump plugin to 1.9.0

Recompute Description/SKILL.md/Directory token counts for every skill
with tiktoken-cli (several skills grew since the last measurement, e.g.
golang-continuous-integration's assets/, golang-how-to's references/).
Also add missing golang-pkg-go-dev to the skills tree diagram.

gemini-extension.json

@@ -1,5 +1,5 @@
 {
   "name": "cc-skills-golang",
-  "version": "1.8.0",
+  "version": "1.9.0",
   "description": "AI Agent Skills for production-ready Go projects"
 }