Drives the user's installed `git` via std::process::Command — the
same approach as the TS app's git-sys backend — so no libgit2/git2 C
dependency enters the workspace.
Covers repo discovery / init, working-tree status, staging, commit,
branch list / create / switch, history + diff, remotes, SSH keys and
credential storage. Adds a worktree-isolated merge orchestrator: a
branch merge runs in a throwaway detached worktree so a conflicting
.op merge never writes conflict markers into the live document — a
clean merge is fast-forwarded back, a conflicting one is reported as
a file-granular ConflictBag with the live tree left pristine.
