Add docstrings to agent-os control plane classes

Package: packages/agent-os/modules/control-plane/src/agent_control_plane/

What

The control plane module has 31+ classes across multiple files with missing or minimal docstrings. These are core architectural components (A2A adapter, MCP adapter, governance layer) that need documentation for contributors and users.

Priority files (pick any one to start):

• a2a_adapter.py — A2AMessageType, A2AAdapter, A2AAgent, A2AMessage
• mcp_adapter.py — MCPMessageType, MCPAdapter, MCPServer, MCPRequest
• governance_layer.py — governance enforcement classes
• control_plane.py — main AgentControlPlane class

Docstring format

Follow Google-style docstrings:

class A2AAdapter:
    """Adapts Agent-to-Agent protocol messages for governance enforcement.
    
    Translates A2A protocol messages into governance-aware operations,
    applying policy checks and trust verification at each message boundary.
    
    Args:
        governance_engine: The policy engine to evaluate against.
        trust_store: Trust score storage backend.
    
    Example:
        adapter = A2AAdapter(engine, store)
        result = adapter.process(message)
    """

Acceptance Criteria

• All public classes in at least one file have docstrings
• Docstrings include: description, Args, Returns, Raises where applicable
• At least one usage Example per class
• pydocstyle passes on modified files

Note: You don't need to do all files — pick one file and submit a PR. Multiple contributors can work on different files in parallel.