Pedagogy: refresh tutorial/10.git.ipynb (drop 2.23 hedges; add restore, SSH, default-branch=main, --rebase-merges)

[kwlee2025cpp]

Why

tutorial/10.git.ipynb was authored when git switch/git restore
were experimental (git 2.23). 6+ years later they are stable, default
branch is main not master, and a few topic gaps have opened up
(SSH-key auth, --rebase-merges). The current intro also jumps
straight into "what is version control" without motivating the
beginner-relevant question — how do I keep my work and not lose it? —
that the tutorial actually answers.

Related:

• Easier ReadMe.md #14 — ReadMe.md cleanup notes "move SW install to tutorial"; this
  issue is a step in that direction for the git portion.
• Maintenance: remove scratch files from repo and tighten .gitignore #387 — .gitignore tightening is the natural sibling for any
  "keep your fork clean" subsection added here.

Concrete edits

• New beginner-framing intro paragraph before the existing
  version-control definition cell. Bilingual (English + Korean) to
  match the rest of the notebook. Frames the three questions git
  answers (save / recover / share).
• Drop (git >= 2.23) qualifier ×4. Four headings still hedge
  git switch / git switch -c. Drop the qualifier; flip ordering so
  switch / restore are primary and checkout <branch> is shown as
  legacy.
• Add git restore subsection alongside git switch:
  • git restore <file> to discard an unstaged edit
  • git restore --staged <file> to unstage
• Default branch name = main. One-liner near the existing
  git config discussion; since git 2.28 (mid-2020), git init -b main
  and init.defaultBranch=main are the convention.
• Add SSH key authentication section alongside the existing
  PAT section:
  • ssh-keygen -t ed25519 -C "you@example.com"
  • add public key to GitHub → Settings → SSH keys
  • verify with ssh -T git@github.com
  • switch existing remotes via
    git remote set-url origin git@github.com:user/repo.git
  • brief contrast: SSH = once-per-machine; PAT = HTTPS bearer
    (still needed for CI / locked-down environments). Both remain
    valid.
• Add git rebase -i --rebase-merges note. Default interactive
  rebase silently drops merge commits; --rebase-merges preserves them
  — relevant for repos with upstream-merge / template-chain commits.

No-touch list (stable, already correct)

clone, add, commit, status, log, diff, remote, push,
pull, fetch, config, branch, basic rebase, fork→sync workflow.

Out of scope (future expansion candidates)

• git maintenance (2.30), git sparse-checkout (2.25), bundle URI
  clone (2.38), git rebase --update-refs (2.38), git subtree (Could git subtree promote bureaucracy? #61),
  pre-commit hooks for ipynb output stripping (git hook code formatting #78). Park for a future
  intermediate tutorial.

Open design calls (resolve during implementation)

• SSH section depth: quick-reference (5 commands, no troubleshooting)
  or full walkthrough including ssh-agent setup and key-types
  rationale (ed25519 vs RSA)?
• Order on the page: PAT-first (existing flow) or SSH-first?