feat(schema): add max_tokens field to AgentDef for per-agent override

Summary

AgentDef has no max_tokens field. The fallback logic in agent_builder.py already exists:

agent_max_tokens = getattr(agent, "max_tokens", None)
max_tokens = agent_max_tokens if agent_max_tokens is not None else default_max_tokens

But since AgentDef uses extra="forbid" and has no max_tokens field, specifying it in YAML causes a validation error, and getattr always returns None.

Use Case

In multi-agent workflows, different agents have different output length requirements:

・Creator agents: need large max_tokens(e.g. 32768) to generate long artifacts.
・Reviewer agents: only produce short reviews, a smaller max_tokens (e.g. 8192) suffices and reduces cost.

Proposal

Add max_tokens to AgentDef in schema.py

max_tokens: int | None = None

This enables per-agent configuration in YAML:

agents:
  - name: creator
    max_tokens: 32768
    prompt: !file prompt_creator.md
  - name: reviewer
    max_tokens: 8192
    prompt: !file prompt_reviewer.md

No logic change needed -- the fallback in agent_builder.py already handles it. Default behavior is unchanged (omitted -> inherits runtime.max_tokens).

Start by reading AgentDef in schema.py and the existing fallback logic in agent_builder.py. Add the field so YAML accepts per-agent max_tokens values, while confirming that omitted values still inherit runtime.max_tokens.