Handles PLAN phase (create/modify) and EXECUTE/REJECT phase.

PLAN phase (when action is not present):

• If stackId provided → MODIFY existing stack
• If no stackId → CREATE new stack
• LLM generates plan, stores in context.v3Plan, waits for approval

EXECUTE/REJECT phase (when action is present):

• If approve → executes via ActionExecutor
• If reject → clears plan and resets phase

Response is Server-Sent Events (SSE) stream.