Validate the objectives in CI, with the StepIt Editor's validator

[kineticsystem]

Summary

• objectives job in ci.yml: builds the StepIt Editor's native validator against the ROS package of BehaviorTree.CPP — the library and the version (4.10.0) the commander loads trees with — and runs validate on src/plugins/stepit_objectives/objectives: the editor's own checks, then the library loading every file with the node models of the rig's behaviors. A broken objective (unknown node or port, missing subtree, bad XML) fails the pull request instead of being refused by the commander on the robot.
• The editor mounts the repo, not the objectives' folder. A bind mount follows the folder, not its path: when gh pr merge switched to a main without src/plugins, git deleted and recreated the folder, and the running editor showed the deleted one, empty. It now mounts the repo at ~/rig, as the commander does, and opens the objectives through BEHAVIORS_DIR.
• stepit-editor 83a2a03 → c4c6d7e (stepit-editor#17): its validator builds against the ROS package, which this job needs.
• README and CLAUDE.md: the job, and how to run the same check by hand.

Test plan

• The objectives job run with act on a clean clone: 10 files, 0 errors, BehaviorTree.CPP loading each.
• With a planted broken objective (unknown node, missing subtree): 3 errors, the job fails.
• The editor recreated with the new mount: it opens the 10 objectives, and validate.sh in its container finds no error.
• The run on GitHub.

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