Enforce max 8 favorites and unfavorite non-runnable objectives

[davetcoleman]

Summary

Cleans up Objective favorite metadata across all config packages to prepare for CI enforcement of favorite limits.

• Added _favorite attribute to all objectives that were missing it, explicitly setting them to false for consistency
• Unfavorited non-runnable objectives — objectives with <Metadata runnable="false"/> should never be favorites since they can't be executed directly
• Unfavorited MPC Pose Tracking objectives (3) and ML Segment objectives (2) to reduce favorite count
• Ran Prettier XML formatting on all modified files to ensure consistent formatting

These changes are required for the new CI check in #517, which enforces:

1. Non-runnable objectives must not be favorited
2. A maximum of 8 favorited objectives per config package — if everything is favorited, nothing is favorited

The end result is lab_sim has these favorites:

Test plan

• Verify non-runnable objectives no longer appear in the favorites list in the UI
• Verify favorited runnable objectives are unaffected
• CI passes after Add pre-commit hooks for Objective favorite and mujoco_viewer validation #517 merges

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