chore: bump version to 3.0.1 + changelog entry

Atomic bump across all four manifests:
- SKILL.md (root)
- skills/last30days/SKILL.md (internal spec)
- .claude-plugin/plugin.json
- gemini-extension.json

CHANGELOG entry documents the skill-upload packaging fix, vendor/ removal,
legacy plans/ removal, and the new scripts/build-skill.sh builder.

gemini-extension.json

@@ -1,6 +1,6 @@
 {
   "name": "last30days-skill",
-  "version": "3.0.0",
+  "version": "3.0.1",
   "description": "Research a topic from the last 30 days across Reddit, X, YouTube, TikTok, Instagram, Hacker News, Polymarket, and the web.",
   "settings": [
     {