DAWE Engine: full phase state machine

[nightjarrr]

Part of #144 (DAWE POC epic).

Replace the POC's linear phase dispatch (#155) with a full state machine:

• Forward transitions (happy path)
• Backward transitions: PO can request returning to an earlier phase for re-iteration (e.g. back to design after unsatisfactory implementation)
• Escalation branches: handoff status-driven paths (escalated → PM on-call, partial → PM on-call)
• Docs-type issue shortcut: phases 1 → 2 → 7 only

Block: MVP.