Trigger selfpatch_demos CI on merge to main

[bburda]

Problem

When changes are merged to main in ros2_medkit, there is no automated verification that the downstream demo images (selfpatch_demos) still build successfully. This has already caused breakage the recent addition of cmake/ROS2MedkitCompat.cmake went undetected because the demo Dockerfiles were not updated to preserve the new directory, and no cross-repo CI caught it (selfpatch/selfpatch_demos#28, selfpatch/selfpatch_demos#30).

The selfpatch_demos CI (ci.yml) already has a docker-build job that builds all three demo images (sensor_diagnostics, turtlebot3_integration, moveit_pick_place), but it only runs on PRs and pushes to selfpatch_demos itself. Changes in ros2_medkit which the demos clone from main at build time are invisible to it.

Proposal

Add a repository_dispatch or workflow_dispatch trigger to selfpatch_demos CI, and dispatch it from ros2_medkit CI on push to main.

Option A: repository_dispatch (recommended)

selfpatch_demos add trigger to existing ci.yml:

on:
  pull_request:
    branches: [main]
  push:
    branches: [main]
  repository_dispatch:
    types: [ros2_medkit_updated]

ros2_medkit add a job to ci.yml that fires after successful build:

notify-demos:
  needs: build-and-test
  if: github.event_name == 'push' && github.ref == 'refs/heads/main'
  runs-on: ubuntu-latest
  steps:
    - name: Trigger selfpatch_demos CI
      uses: peter-evans/repository-dispatch@v3
      with:
        token: ${{ secrets.DEMOS_DISPATCH_TOKEN }}
        repository: selfpatch/selfpatch_demos
        event-type: ros2_medkit_updated

Requires a PAT or fine-grained token with actions:write on selfpatch_demos, stored as DEMOS_DISPATCH_TOKEN in ros2_medkit secrets.

Option B: workflow_dispatch + GitHub CLI

Same idea but using gh workflow run from ros2_medkit CI. Slightly simpler but less structured.

Acceptance criteria

• A merge to ros2_medkit/main triggers a full docker-build job in selfpatch_demos
• Demo build failures are visible in ros2_medkit CI (or at minimum in selfpatch_demos with a clear link back)
• No impact on selfpatch_demos PR workflows dispatch builds should not block unrelated PRs