feat: advanced agentic capabilities

[zseta]

Work In Progress

This is going to be a bigger change to add autonomous capabilities to the agent. In practice, expected new features:

• per-step worker agent
• per-step QA agent that can instruct the worker agent to refine output
• instead of passing variables across steps linearly, let's use a shared state "object", that is also persisted on disk, accessible and modifiable by each step's worker

Out of scope here, but would be nice:

• process improvement agent: check the history of sessions and suggest ways to replace AI with deterministic scripts, and find other ways to reduce token usage

Done so far:

• Complete sub-workflow as-step support 0bd648f
• Worker and QA playbooks, replaced I/O system with langgraph-style state management 3a51d67