feat: remove hardcoded default models from integrations

[nicoloboschi]

Summary

• Remove hardcoded defaultModel from openclaw and claude-code integrations
• Integrations now rely on Hindsight API's built-in default models instead of each integration maintaining its own model list
• Updated integration docs to reflect the change

Changes

• openclaw (index.ts): stripped defaultModel from PROVIDER_DETECTION, removed all providerInfo?.defaultModel fallbacks in detectLLMConfig
• claude-code (llm.py): same change in the Python port
• docs: removed default model column from openclaw provider table, removed model hints from setup comments in both openclaw and claude-code docs

Test plan

• Verify openclaw plugin starts correctly with just OPENAI_API_KEY set (no explicit model)
• Verify claude-code integration starts correctly with just a provider API key
• Verify explicit HINDSIGHT_API_LLM_MODEL override still works