feat(terminal): real git branches, merges and conflicts, matching Git 2.56

[thierryvm]

Why

A branch in the simulator was only a name: switching changed no file, git merge recorded a fake merge commit, and the conflicts lesson never showed a conflict. Git is now modelled as a commit graph, and every output was compared with Git 2.56.

What changes

• Commit graph (commands/gitHistory.ts, new): commits with parents and dates, branches and tags as refs, merge base, log order, git's three-way merge with conflict markers, the --stat graph as diff.c lays it out, and object ids computed like git (tree, commit, annotated tag). Same files, author, date and message give the same id as a real repository — checked against real git.
• Branches: git branch (list, create, -d/-D, -m, -v, -a), git switch / git checkout update the working tree, carry local changes (M\tfile first) or refuse with git's messages (local changes, untracked files in the way), switch -, tracking branches from origin/*, path checkout (Updated N paths from the index / from a tree id, silent with --, --ours/--theirs).
• Merges: fast-forward, --no-ff, --ff-only (with git's hints), --squash, three-way merge commits (Merge branch 'x' [into y]), content / add/add / modify/delete conflicts, --abort, --continue, every refusal during a merge; git status during a merge (long form and UU/AA/UD/DU); git add / git rm resolve; git commit concludes.
• History: git log real format with decorations, --oneline, -n forms, --all, --author, --grep, --stat, -p, ranges, paths, --graph on a linear history; git show (patch, --stat, merges, rev:path, annotated tags); git tag; git reset on the graph; git cherry-pick and git rebase replay commits for real. Rebase -i, cherry-pick/rebase conflicts, detached HEAD and the --graph drawing of forks say "not simulated" instead of inventing output.
• Comparisons and errors: git diff A B, A..B, A...B (from the merge base) and --stat; git diff --staged during a conflict (* Unmerged path); git tag -n, --contains/--points-at/--merged, -f (Updated tag); git rebase <upstream> <branch>; invalid branch/tag names, unknown options (with the usage text generated from Git 2.56 in commands/gitUsage.ts), switch/checkout argument errors, merge -, merge --abort <arg>, and a refused three-way merge (Merge with strategy ort failed., exit 2).
• Messages and paragraphs: several -m make paragraphs (commit, tag); a conflicted merge proposes # Conflicts: comments, removed by the editor path (git commit, merge --continue) and kept by --no-edit, as git does. An option git lists in its usage but the simulator does not do now says "not simulated" instead of "unknown option".
• Lessons: gitRepoWithBranch now has the branch one commit ahead (the merge lesson is a real fast-forward, Updating a3f8c12..b7e2d45); new gitRepoWithConflict gives the conflicts lesson a real conflict on <title>; the rename example in git-branch and the branch comparison in git-diff-gitignore are replayable.
• Reference replay: the git examples run in a repository with a real graph (main with history, feature/login holding the commits the cherry-pick examples name).

Verification

• Real git: Git 2.56 in throwaway sandboxes (no network, no editor, no pager); stdout/stderr order checked under a pseudo-terminal (WSL script), since redirected captures reorder lines.
• src/test/gitBranches.test.ts (56 tests, real values, including ids 52f1600…, merge 6bdf3ce…, tag 6616dc3…); old engine tests that encoded the fake behaviour rewritten.
• type-check, lint, full suite (2901 pass; the 3 live-Supabase integration tests timed out once under load and passed on two isolated reruns), build; theory gaps 148 → 148, bash-on-Windows 34 → 34.
• Gates: curriculum-validator (0 CRITICAL), test-runner, terminal-fidelity-auditor (two passes: the first found 14 families of differences, the second 11 more — among them git tag -l -d silently deleting a tag; all fixed, each expected value re-captured from real git), feature-dev:code-reviewer (two passes: git restore on a conflicted file, git show on a merge fixed by hand, switch -f accepted but ignored, options git knows reported as unknown — all fixed).
• Chrome (local): conflicts lesson desktop 1280 and mobile, no overflow, no console error.

Also fixes the date of the #399 changelog/story entries (29 September, not 30).

Known follow-ups

• Exercises are still validated on the typed command and auto-advance after 2.5 s: the learner leaves the conflicts lesson before seeing the conflict. Next PR: multi-step exercises validated on the terminal state.
• git checkout -t origin/<branch> (low reach: remoteRefs only holds origin/<current branch>).
• git log --graph with --grep/--author (git shifts columns around skipped commits) and for forked histories, cherry-pick/rebase conflict states, detached HEAD, commit --amend.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated
terminal-learning	Ready	Preview	Sep 29, 2026 8:42pm UTC