#3 Git reader module (log/tree/show/diff/bundle/tarball)

Context: All pages need git data via CLI (no libgit2). Shared module used by code/commits/releases/bundle. Tasks:

• Module gitx: resolve_branch(repo, flag|env|main->master), rev_parse_short, log_n(100): sha,subject,author,date, ls_tree(branch, path), show(branch:path)->bytes, diff(sha)->unified text (capped, binary-aware).
• Shells out to git -C <repo> with timeout; non-UTF8 paths handled (lossy display + raw still exact); errors are typed, never panic.
• snapshot(repo, branch, out): git bundle create repo.bundle <branch> + repo.tar.gz via git archive.
• Unit tests with a temp fixture repo (init, 3 commits, branch, binary file). Success rubric:
• Tests green on fixture: log order, ls-tree dirs-first sort, show byte-exact, diff contains expected hunk, bundle clones (git clone repo.bundle works), tarball extracts.
• Missing branch -> loud error naming tried names. Non-repo path -> clean error, exit non-zero.
• No git command exceeds 10s (timeout tested with mocked GIT ceiling? at least timeout plumbed). Swarm: Owl/Platinum. Branch m1/gitx. No HTML in this issue.