[pull] master from flypythoncom:master - #12

- Inject progressive per-checkpoint hints into all 40 checkpoint pairs
  (80 EN/ZH lesson files); DA lessons gain full frontmatter so they
  enter the content manifest like the other courses.
- paths/foundation/path.json declares the first-agent branch group so
  the site can treat Claude Code / Codex modules as alternatives.
- Record real-agent solvability runs (Devin, SWE-2 Max) in all 8 course
  REVIEW.md files — challenge-mode runs with observed deviations noted;
  starters restored and --expect-failure re-verified.
- Regenerate content-manifest.json; update repo-plan status lines.
- Verified: verify_courses (8/8), verify_examples (5/5), manifest,
  catalog export, and readme rendering checks all green.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>