Add PersonSkeleton common type and skeleton field for Occupant

[jdsika]

Reference to a related issue in the repository

Extends the Occupant message to support skeleton/posture data for vehicle occupants (e.g. for driver monitoring / DCAS use cases).

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What is this change?

Adds an \Occupant.Skeleton\ message in \osi_occupant.proto\ that reuses \MovingObject.PedestrianAttributes.Bone\ directly, giving occupants a skeleton representation analogous to what pedestrians already have.

Changes:

• **\osi_occupant.proto**: New \Occupant.Skeleton\ wrapper message with \�bcenter_to_root\ (offset from vehicle bounding box center) and
  epeated MovingObject.PedestrianAttributes.Bone skeleton_bone. New import of \osi_object.proto.
• **\osi_object.proto**: Migration note on \PedestrianAttributes\ documenting that \Bone\ is reused by \Occupant::Skeleton\ and that a common skeleton type is planned for OSI 4.
• **.github/copilot-instructions.md**: New Copilot context file documenting build/test commands, architecture, and conventions.

Does it break any existing functionality?

No. All changes are purely additive. Zero type duplication — \PedestrianAttributes.Bone\ is referenced directly.

How has it been tested?

• Proto compilation validated with \grpc_tools.protoc\
• All existing tests pass: \python -m unittest discover tests\ (24 passed, 651 subtests)

Checklist

• My suggestion follows the style and contributors guidelines.
• I have taken care about the message documentation and the fields and enums documentation.
• I have done the DCO signoff.
• My changes generate no errors when passing CI tests.
• I have successfully implemented and tested my fix/feature locally.
• Appropriate reviewer(s) are assigned.

[pmai]

Would it not be easier to just re-use either PersonAttributes or PersonAttributes::Bone directly for now? This should be possible, as we have done so in the past. Maybe cannot be done in osi_common (potential circular includes, I would imagine), but directly in osi_occupant this should be possible...