feat(shapes): keypoint annotation with skeleton definitions

Pose/keypoint labeling is a common request that currently has no first-class support — users approximate it with points and group_id by hand.

Proposal: a keypoint mode driven by a skeleton definition file (keypoint names, order, edges, per-point visibility flag). The UI steps through the named points for each instance; edges render between placed points; instances link via group_id.

Scope sketch:

• Skeleton config: YAML/JSON with names, connections, colors.
• Visibility states per keypoint (visible / occluded / absent).
• Output stays Labelme JSON (points + group_id + a skeleton reference), so existing tooling keeps working.