Implement navigation-based workflow engine with server-side enforcement

[m2ux]

Summary

Replace the current agent-interpreted workflow execution model with a server-driven navigation engine that enforces workflow fidelity deterministically.

Problem

Currently, agents interpret workflow schemas (.toon files) and are responsible for:

• Parsing and understanding workflow structure
• Maintaining execution state
• Enforcing checkpoints and loop semantics
• Following step sequences

This leads to workflow fidelity issues where agents:

• Batch operations instead of executing step-by-step
• Skip checkpoints or assume responses
• Optimize for speed over process compliance
• Interpret semantics inconsistently across invocations

Proposed Solution

Implement a navigation-based workflow engine where:

1. Engine owns semantics - Server-side enforcement of checkpoints, loops, and transitions
2. Agent traverses landscape - Agent receives valid actions, chooses from options, reports completion
3. Opaque state tokens - State is encoded/compressed so agents cannot interpret or be biased by it
4. Stateless engine - State passed in/out on each call; agent stores for resumption

Key API Changes

• start_workflow(workflow_id) → returns position, available actions, opaque state
• get_position(workflow_id, state) → returns current position
• get_available_actions(workflow_id, state) → returns required/optional/blocked actions
• complete_step(workflow_id, state, step_id, summary) → validates and advances state
• respond_to_checkpoint(workflow_id, state, checkpoint_id, option_id) → records response, advances

Benefits

• Deterministic workflow execution
• Checkpoints cannot be skipped
• Loops cannot be batched
• State is tamper-resistant
• Resumption is reliable

Acceptance Criteria

• Navigation API implemented with position-aware responses
• State tokens are opaque (compressed/encoded)
• Checkpoints block progression until responded
• Loop iterations enforced sequentially
• Invalid transitions rejected with clear errors
• Existing workflows continue to function
• Documentation updated