chore: clean up GitHub Actions workflows

- Rename dotnet.yml → build-dotnet.yml for clarity
- Rename publish-images.yml → publish-docker-images.yml for clarity
- Rename strip-dev-files-on-main.yml → cleanup-dev-files.yml for clarity
- Delete squad-heartbeat.yml (auto-triage agent not needed)
- Delete squad-issue-assign.yml (team routing not needed)
- Delete squad-triage.yml (lead triage not needed)
- Delete sync-squad-labels.yml (squad label sync not needed)

Remaining workflows: build, publish Docker images, and dev file cleanup.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: fboucher