pi-synthetic: Pi 0.77.0 compatibility and model metadata refresh
Removed models no longer returned by Synthetic (hf:zai-org/GLM-5, hf:deepseek-ai/DeepSeek-V3.2), added hf:Qwen/Qwen3.6-27B, and updated syn:small:vision to point at the new Qwen target.
- Update Pi package metadata and docs for Pi 0.77.0 compatibility
- Use Pi 0.77 environment interpolation for the Synthetic provider API key fallback
- Refresh Synthetic model metadata from the live API
- Register the Synthetic web search tool with Pi's current defineTool helper