feat(categories): change quick category default model from claude-haiku-4-5 to gpt-5.4-mini

GPT-5.4-mini provides stronger reasoning at comparable speed and cost.
Haiku remains as the next fallback priority in the chain.

Changes:
- DEFAULT_CATEGORIES quick model: anthropic/claude-haiku-4-5 → openai/gpt-5.4-mini
- Fallback chain: gpt-5.4-mini → haiku → gemini-3-flash → minimax-m2.5 → gpt-5-nano
- OpenAI-only catalog: quick uses gpt-5.4-mini directly
- Think-mode: add gpt-5-4-mini and gpt-5-4-nano high variants
- Update all documentation references

docs/guide/installation.md

@@ -287,6 +287,7 @@ Not all models behave the same way. Understanding which models are "similar" hel
 | ----------------- | -------------------------------- | ------------------------------------------------- |
 | **GPT-5.3-codex** | openai, github-copilot, opencode | Deep coding powerhouse. Required for Hephaestus.  |
 | **GPT-5.4**       | openai, github-copilot, opencode | High intelligence. Default for Oracle.            |
+| **GPT-5.4 Mini**  | openai, github-copilot, opencode | Fast + strong reasoning. Default for quick category.     |
 | **GPT-5-Nano**    | opencode                         | Ultra-cheap, fast. Good for simple utility tasks. |
 
 **Different-Behavior Models**: