docs: add model recommendations documentation

- Create docs/MODEL-RECOMMENDATIONS.md with task-type guidance
- Update docs/CONFIGURATION.md with model configuration section
- Update CLAUDE.md with agent model configuration overview

Fixes #302

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/CONFIGURATION.md

@@ -522,6 +522,56 @@ cat .env
 
 ---
 
+## Agent Model Configuration
+
+Agents can specify which Claude model to use for optimal cost/performance.
+
+### Model Options
+
+| Model | Use For | Cost |
+|-------|---------|------|
+| `opus` | Complex reasoning, security analysis | Highest |
+| `sonnet` | Implementation, coordination (default) | Medium |
+| `haiku` | Simple validation, quick checks | Lowest |
+
+### Configuration Levels
+
+**1. Agent-Level (highest priority)**
+
+Add to agent frontmatter in `agents/*.md`:
+
+```yaml
+---
+name: planner
+description: Sprint planning agent
+model: opus
+---
+```
+
+**2. Plugin-Level (fallback)**
+
+Add to `plugin.json`:
+
+```json
+{
+  "defaultModel": "sonnet"
+}
+```
+
+**3. System Default**
+
+If neither is specified, agents use `sonnet`.
+
+### Inheritance Chain
+
+```
+Agent model → Plugin defaultModel → System default (sonnet)
+```
+
+See [Model Recommendations](MODEL-RECOMMENDATIONS.md) for detailed guidance on model selection by task type.
+
+---
+
 ## Security Best Practices
 
 1. **Never commit tokens**