A real system prompt and prompt caching #12

Do.

• System prompt: cwd, OS, shell, date, git branch/status/recent commits, tool guidance. Byte-stable across turns.
• Anthropic cache_control on system, tools and the last messages; OpenAI prompt_cache_key = session id. Report cache tokens in usage.
• Drop hard-coded temperature: 0.1 for models that reject it (OpenAI reasoning models); make max_tokens per model.

Spec §5.1 #7–8.

Spec: SPEC_MUXCODE_HARNESS_AND_FEATURE_GAPS_2026_09_26.md