feat: add 'add-model' skill for adding new LLM models (#262)

Co-authored-by: Letta <noreply@letta.com>

.skills/add-model/SKILL.md

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+---
+name: Add New Model
+description: Guide for adding new LLM models to Letta Code. Use when the user wants to add support for a new model, needs to know valid model handles, or wants to update the model configuration. Covers models.json configuration, CI test matrix, and handle validation.
+---
+
+# Add New Model
+
+This skill guides you through adding a new LLM model to Letta Code.
+
+## Quick Reference
+
+**Key files**:
+- `src/models.json` - Model definitions (required)
+- `.github/workflows/ci.yml` - CI test matrix (optional)
+- `src/tools/manager.ts` - Toolset detection logic (rarely needed)
+
+## Workflow
+
+### Step 1: Find Valid Model Handles
+
+Query the Letta API to see available models:
+
+```bash
+curl -s https://api.letta.com/v1/models/ | jq '.[] | .handle'
+```
+
+Or filter by provider:
+```bash
+curl -s https://api.letta.com/v1/models/ | jq '.[] | select(.handle | startswith("google_ai/")) | .handle'
+```
+
+Common provider prefixes:
+- `anthropic/` - Claude models
+- `openai/` - GPT models  
+- `google_ai/` - Gemini models
+- `google_vertex/` - Vertex AI
+- `openrouter/` - Various providers
+
+### Step 2: Add to models.json
+
+Add an entry to `src/models.json`:
+
+```json
+{
+  "id": "model-shortname",
+  "handle": "provider/model-name",
+  "label": "Human Readable Name",
+  "description": "Brief description of the model",
+  "isFeatured": true,  // Optional: shows in featured list
+  "updateArgs": {
+    "context_window": 180000,
+    "temperature": 1.0  // Optional: provider-specific settings
+  }
+}
+```
+
+**Field reference**:
+- `id`: Short identifier used with `--model` flag (e.g., `gemini-3-flash`)
+- `handle`: Full provider/model path from the API (e.g., `google_ai/gemini-3-flash-preview`)
+- `label`: Display name in model selector
+- `description`: Brief description shown in selector
+- `isFeatured`: If true, appears in featured models section
+- `updateArgs`: Model-specific configuration (context window, temperature, reasoning settings, etc.)
+
+**Provider prefixes**:
+- `anthropic/` - Anthropic (Claude models)
+- `openai/` - OpenAI (GPT models)
+- `google_ai/` - Google AI (Gemini models)
+- `google_vertex/` - Google Vertex AI
+- `openrouter/` - OpenRouter (various providers)
+
+### Step 3: Test the Model
+
+Test with headless mode:
+
+```bash
+bun run src/index.ts --new --model <model-id> -p "hi, what model are you?"
+```
+
+Example:
+```bash
+bun run src/index.ts --new --model gemini-3-flash -p "hi, what model are you?"
+```
+
+### Step 4: Add to CI Test Matrix (Optional)
+
+To include the model in automated testing, add it to `.github/workflows/ci.yml`:
+
+```yaml
+# Find the headless job matrix around line 122
+model: [gpt-5-minimal, gpt-4.1, sonnet-4.5, gemini-pro, your-new-model, glm-4.6, haiku]
+```
+
+## Toolset Detection
+
+Models are automatically assigned toolsets based on provider:
+- `openai/*` → `codex` toolset
+- `google_ai/*` or `google_vertex/*` → `gemini` toolset
+- Others → `default` toolset
+
+This is handled by `isGeminiModel()` and `isOpenAIModel()` in `src/tools/manager.ts`. You typically don't need to modify this unless adding a new provider.
+
+## Common Issues
+
+**"Handle not found" error**: The model handle is incorrect. Run the validation script to see valid handles.
+
+**Model works but wrong toolset**: Check `src/tools/manager.ts` to ensure the provider prefix is recognized.