feat(FN-980): add mission autopilot for autonomous slice progression

- Add MissionAutopilot class with state machine (inactive → watching → activating → completing)
- Add autopilot database schema: autopilotEnabled, autopilotState, lastAutopilotActivityAt columns
- Integrate autopilot with scheduler: handleTaskCompletion() triggers slice activation
- Add API routes: GET/PATCH /missions/:id/autopilot, POST start/stop endpoints
- Add autopilot toggle UI in MissionManager dashboard component
- Add retry logic with exponential backoff (up to 3 attempts) for slice activation
- Add background poll (60s) to detect stale autopilot missions
- Include changeset for @gsxdsm/fusion minor bump

.changeset/fn-980-mission-autopilot.md

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+---
+"@gsxdsm/fusion": minor
+---
+
+Add Mission Autopilot mode for autonomous mission progression.
+
+Missions in autopilot mode automatically:
+- Activate slices when previous ones complete
+- Create tasks from features when slices are activated
+- Progress through milestones as work completes
+- Track overall mission health and state