Add overlap-warning workflow for files shared with moveit_pro_empty_ws

[JWhitleyWork]

Problem

Files that exist in both moveit_pro_empty_ws (parent) and moveit_pro_example_ws (fork) can drift when a change is made only to the fork. Reviewers don't always notice during PR review, and the divergence is usually discovered later during an upstream merge — at which point the fix is much more disruptive.

Alternatives considered

1. Sync a manifest of upstream paths into this repo via a periodic workflow in moveit_pro_empty_ws. Rejected: introduces another moving part and the manifest goes stale between syncs.
2. git clone moveit_pro_empty_ws inside the action and diff locally. Rejected: slower, requires auth handling for a private/internal repo, and is overkill — we only need a path list, not file contents.
3. Chosen: read moveit_pro_empty_ws's tree from the GitHub API at PR time. A single recursive git/trees call returns every path, the data is always current, and no checkout of either repo is needed.

Approach

One actions/github-script@v9.0.0 step. No actions/checkout. The script:

• Lists PR files via pulls.listFiles (paginated, ignoring deletions).
• Fetches moveit_pro_empty_ws's default-branch tree via git.getTree with recursive: true.
• Intersects the two path sets.
• Posts, updates, or deletes a sticky comment keyed by an HTML marker (<!-- empty-ws-overlap-warning -->), so subsequent pushes update one comment instead of spamming the PR. If a later push removes the overlap entirely, the comment is deleted.
• Emits a core.warning if the upstream tree response is ever truncated (current size is well under GitHub's ~100k-entry limit, so this is precautionary).

Tradeoffs

• The warning is advisory only — it never blocks a PR. Sometimes a fork-only change is the correct choice (e.g., example-specific tuning of a shared config), so failing the check would create false-positive friction.
• Uses the default GITHUB_TOKEN. This works because both repos are in the same org and visible to the workflow. If moveit_pro_empty_ws access ever tightens, the token can be swapped for a fine-grained PAT secret without other changes.
• Action pins use the actions/checkout@<SHA> # v<version> style used elsewhere in ci.yaml.

[Copilot]

Pull request overview

Adds a new GitHub Actions workflow that warns reviewers when a PR touches files that also exist in the upstream moveit_pro_empty_ws repository, using a sticky PR comment that is created, updated, or deleted on subsequent pushes.

Changes:

• New workflow warn-empty-ws-overlap.yaml triggered on pull_request events.
• Uses actions/github-script to list changed files, fetch upstream's recursive tree via the GitHub API, and intersect the two path sets.
• Manages a single sticky comment keyed by an HTML marker, and emits a core.warning on overlap or on truncated upstream tree responses.

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