Document Flow model fields in AGENTS.md § 5

[dgenio]

Context

AGENTS.md § 5 (Executor and flow semantics) documents FlowStep.input_mapping, ExecutionResult, and StepRecord, but does not document the Flow model fields themselves. The following fields in chainweaver/flow.py are undocumented in § 5:

Field	Type	Default	Purpose
deterministic	bool	True	When True, the executor guarantees no LLM calls between steps
trigger_conditions	dict[str, Any] | None	None	Free-form metadata for higher-level orchestrators; ChainWeaver itself does not evaluate these
input_schema	type[BaseModel] | None	None	Optional Pydantic schema for validating initial_input before the first step
output_schema	type[BaseModel] | None	None	Optional Pydantic schema for validating the final merged context after the last step

Proposed change

Add a ### Flow (Pydantic model) subsection to AGENTS.md § 5 documenting these fields, similar to the existing ExecutionResult and StepRecord tables.

Origin

Raised during review of #94 (docs rewrite PR).