Four-bucket model + add/commit loop established
RT completed Lesson 1's hands-on: the sandbox repo has a real commit
(1d96ff8 "add notes"), meaning they ran git init → git add → git commit
themselves, not just read it.
Evidence: committed file present in ./sandbox/, working tree clean.
Implications: buckets 1→2→3 (working dir → staging → local history) and the
verbs add / commit / status can be treated as known floor. Next zone of
proximal development is bucket 4 — the remote: push, pull, origin, and
connecting a local repo to GitHub. That is Lesson 2.