feat: add STRIX_REASONING_EFFORT env var to control thinking effort

- Add configurable reasoning effort via environment variable
- Default to "high", but use "medium" for quick scan mode
- Document in README and interface error panel

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

README.md

@@ -213,6 +213,7 @@ export LLM_API_KEY="your-api-key"
 # Optional
 export LLM_API_BASE="your-api-base-url"  # if using a local model, e.g. Ollama, LMStudio
 export PERPLEXITY_API_KEY="your-api-key"  # for search capabilities
+export STRIX_REASONING_EFFORT="high"  # control thinking effort (default: high, quick scan: medium)
 ```
 
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