mix just_bash.audit (JustBash v0.4.0)

Scans Elixir source for filesystem call shapes that the 0.3 → 0.4 migration broke in ways the compiler cannot catch (see UPGRADING.md). The legacy patterns still compile and then misbehave at runtime:

Rule	Legacy shape	Failure mode in 0.4
`legacy_module`	`JustBash.Fs` / `InMemoryFs` reference	compile error (undefined module)
`stale_ok_tuple`	`{:ok, x}` matched on a read	silently falls through to the error clause
`atom_error`	`{:error, :enoent}` clause	never matches; falls to catch-all or crashes
`exists_truthy`	`FS.exists?/2` in a condition	tuple is always truthy — branch always taken
`legacy_opt`	`mkdir recursive:` / `rm force:`	raises `ArgumentError` at runtime
`stat_boolean_field`	`stat.is_file` etc.	`KeyError` at runtime
`fs_data_access`	`bash.fs.data`	`KeyError` — `bash.fs` is a `%VFS{}`

Calls are recognized on the JustBash.FS module, a discovered alias JustBash.FS[, as: ...], or the bare FS name (heuristic).

mix just_bash.audit             # scans lib/
mix just_bash.audit lib test    # scans multiple paths
mix just_bash.audit path/to/file.ex

Prints one line per finding (file:line [rule] message) and exits non-zero when anything is found, so it can gate CI.

This task exists for the 0.3 → 0.4 migration window and will be removed once the legacy shapes are gone.