feat: configure Anthropic prompt-cache TTL in settings.yaml

Problem

--anthropic-cache-ttl is currently the only way to select Anthropic prompt-cache retention. Its unset default omits cache_control.ttl, which leaves Anthropic at its 5-minute API default. Operators cannot set this persistently in settings.yaml.

Proposed change

• Add an operator-tier anthropic_cache_ttl setting to settings.yaml.
• Accept only 5m and 1h; invalid values should retain the existing warn-and-fall-back behavior.
• Make 1h the effective default when neither the setting nor the CLI supplies a value.
• Define precedence explicitly: CLI flag should override the operator setting.
• Continue stamping the resolved TTL uniformly on every Anthropic cache_control breakpoint.
• Update flag/config/user documentation and tests for precedence, validation, and default wire output.

Scope

Anthropic only; OpenAI/OpenRouter/openaichat cache dialects must remain unchanged.