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>

src/recommended-models.json

@@ -1,14 +1,14 @@
 [
     {
-        "id": "openai/gpt-5.3-codex",
-        "name": "GPT 5.3 Codex",
-        "description": "Coding-focused GPT-5.3 variant with optimized routing.",
+        "id": "openai/gpt-5.5",
+        "name": "GPT 5.5",
+        "description": "OpenAI's GPT-5.5 with strong reasoning and agentic tool use.",
         "context_length": 400000,
         "max_output_tokens": 128000,
         "credits_per_request": 4.921875,
         "provider": "OpenAI",
         "quickLabel": "GPT",
-        "tierModels": { "sonnet": "openai/gpt-5.3-codex", "opus": "openai/gpt-5.3-codex", "haiku": "openai/gpt-5.1-codex-mini" }
+        "tierModels": { "sonnet": "openai/gpt-5.5", "opus": "openai/gpt-5.5", "haiku": "openai/gpt-5.4-mini" }
     },
     {
         "id": "google/gemini-3.1-pro-preview",