Release 2.1.0: sponsor button, GPT 5.5 model, resolver fixes

- Add Sponsor button via .github/FUNDING.yml and package.json sponsor field (GitHub Sponsors)
- Default GPT quick-model to mainline GPT 5.5 (was GPT 5.3 Codex)
- Retarget OpenAI resolver to gpt-<ver> / gpt-<ver>-mini slugs instead of -codex variants
- Fix non-existent gpt-5.5-mini fallback to gpt-5.4-mini
- Bump version to 2.1.0 and update CHANGELOG

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -2,7 +2,7 @@
   "name": "claude-code-chat",
   "displayName": "Chat for Claude Code",
   "description": "Beautiful Claude Code Chat Interface for VS Code",
-  "version": "2.0.9",
+  "version": "2.1.0",
   "publisher": "AndrePimenta",
   "author": "Andre Pimenta",
   "repository": {
@@ -10,6 +10,9 @@
     "url": "https://github.com/andrepimenta/claude-code-chat"
   },
   "license": "SEE LICENSE IN LICENSE",
+  "sponsor": {
+    "url": "https://github.com/sponsors/andrepimenta"
+  },
   "engines": {
     "vscode": "^1.94.0"
   },