Migrate scripts from filesystem auto-discovery to manifest entries

[bburda]

Summary

Scripts API support in #42 uses filesystem auto-discovery (scripts_dir/{entity_id}/{script_id}/metadata.json + script.bash). This works but scripts are managed=false (deletable via API).

Once selfpatch/ros2_medkit#303 lands (scripts section in entity manifest), migrate all 3 demos to use manifest-defined scripts for managed=true (immutable) semantics.

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Proposed solution

1. Add scripts: section to each demo's entity manifest YAML (sensor_manifest.yaml, turtlebot3_manifest.yaml, panda_manifest.yaml)
2. Remove container_scripts/ directories from Dockerfiles (scripts defined in manifest, paths point to /opt/scripts/)
3. Update medkit_params.yaml - may no longer need scripts_dir if all scripts are manifest-defined

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Additional context

Depends on: selfpatch/ros2_medkit#303
Current implementation: filesystem auto-discovery in container_scripts/ dirs (PR from #42)
TODO markers in code reference this issue.