#769 Codify engine/apps boundary + relocate use-case DSL templates out of served gallery

Holistic review (3 parallel reviewers: engine, api, enterprise) found the architecture clean: engine 100 percent generic primitives (67 tools, no vendor code), api blueprints/reactions/oauth/management generic, enterprise vendor/business logic correctly in apps. Only finding: 9 use-case-specific workflow templates sitting in the served gallery api/dsl_templates (eod_banking_etl, newsapi_search, bug_report_workflow, gutenberg_summarizer_workflow, llm_story_summarizer x2, url_summarizer x3) that signal built-in features Highway should not claim. ACTIONS: (A) added repo-root CONTRIBUTING.md (Engine vs Apps boundary + PR checklist) and a NON-NEGOTIABLE boundary rule in CLAUDE.md. (B) git mv the 9 use-case templates into api/dsl_templates/use_cases/ (excluded from the served gallery, which globs top-level dsl_templates/*.py only; the recursive dev runner still validates them) + a README. Generic PATTERN templates remain top-level/served. Severity low (inert examples, not live endpoints like the reverted campaign).