refactor(FN-1353): simplify mission autopilot to single toggle

- Deprecate autoAdvance field in favor of autopilotEnabled as the sole control
- Remove autoAdvance guard logic from MissionAutopilot engine class
- Simplify MissionManager UI to use single autopilot toggle with visual state indicator
- Update MissionAutopilot tests to use autopilotEnabled instead of autoAdvance
- Update MissionManager component tests for simplified UI
- Update AGENTS.md documentation to reflect the simplified autopilot model

packages/core/src/mission-types.ts

@@ -116,7 +116,11 @@ export interface Mission {
   status: MissionStatus;
   /** State of the AI specification interview process */
   interviewState: InterviewState;
-  /** When true, automatically activate the next pending slice when current slice completes */
+  /**
+   * @deprecated Superseded by `autopilotEnabled`. Kept for backward compatibility
+   * with existing mission data. Autopilot now always auto-advances slices when
+   * enabled and watching.
+   */
   autoAdvance?: boolean;
   /** When true, enable autopilot monitoring system for this mission */
   autopilotEnabled?: boolean;