fix: mkdir and rm report backend errors instead of crashing the exec

[ivarvong]

Running mkdir or rm on a mount whose backend rejects the mutation crashed the entire JustBash.exec/2 with a CaseClauseError instead of failing the command:

bash = JustBash.new() |> JustBash.mount("/work", Exgit.Workspace.open(repo))
JustBash.exec(bash, "mkdir -p /work/data")   # ** (CaseClauseError) — git trees cannot store empty directories

Both commands matched only the error kinds the in-memory backend can produce (:eexist/:enoent for mkdir, :enoent/:enotempty for rm). Any other backend error — :enotsup from a git-tree-backed workspace, :erofs from a read-only archive — now reports through FS.strerror/1 with exit code 1, matching how ln already degrades on backends without symlink support.

Adds a NoEmptyDirsBackend fixture in test/support: a writable backend that rejects directory creation (the shape of any git-tree-backed mount), with a read_only mode for :erofs paths. The other mutation commands (cp, mv, tee, touch, ln, chmod, chown) already have catch-all error clauses.

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https://claude.ai/code/session_01FpxHS5iGm9ktgpnL9QZu9x