DAWE: AA agent definition update

[nightjarrr]

Part of #144 (DAWE POC epic).

DAWE-specific updates to the AA agent definition. Primary implementation tracked in #71.

Updates required for DAWE:

• Remove relay-protocol framing (AA is now a direct foreground session with PO, not a subagent)
• Handoff artifact writing behavior (replacing the 'final response' terminal output pattern)
• Dispatch brief reading (engine provides context at session start)
• Communication model update: direct foreground session, no relay