branch executor: doc consolidation, future TODOs; remove redundant demo configs

- Remove branch_mvp / branch_mvp_ray / branch_mvp_ray_pure yamls (configs live in docs + dj-hub)
- Consolidate MVP examples in BranchExecution.md; point to dj-hub for full configs
- Add Future TODOs in branch_executor.py (DAG mixin, Ray resources, validation, tests)
- Keep demos/branch_execution_example.yaml as single reference

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: yxdyc