New features:
- Prompt builder: structured multi-section prompt composition
- .static() alias for cacheable static_instruction
- .inject_context() for dynamic context injection via before_model_callback
- Agent("name", "model") shorthand constructor
- Optional constructor args support in generator pipeline
- Comprehensive README with builder API + operator docs side-by-side

Fixes:
- Auto-convert Prompt objects to strings in _prepare_build_config
- Generator supports optional_constructor_args in seed.manual.toml