feat: add fallback_model input to enable automatic model fallback (#228)

- Add fallback_model input to action.yml matching claude-code-base-action
- Pass fallback_model through to the base action
- Document the new input in README.md inputs table
- Enables automatic fallback when primary model is unavailable

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

action.yml

@@ -31,6 +31,9 @@ inputs:
   anthropic_model:
     description: "DEPRECATED: Use 'model' instead. Model to use (provider-specific format required for Bedrock/Vertex)"
     required: false
+  fallback_model:
+    description: "Enable automatic fallback to specified model when primary model is unavailable"
+    required: false
   allowed_tools:
     description: "Additional tools for Claude to use (the base GitHub tools will always be included)"
     required: false
@@ -133,6 +136,7 @@ runs:
         timeout_minutes: ${{ inputs.timeout_minutes }}
         max_turns: ${{ inputs.max_turns }}
         model: ${{ inputs.model || inputs.anthropic_model }}
+        fallback_model: ${{ inputs.fallback_model }}
         mcp_config: ${{ steps.prepare.outputs.mcp_config }}
         use_bedrock: ${{ inputs.use_bedrock }}
         use_vertex: ${{ inputs.use_vertex }}