cli: exempt ps from the startup proot probe

ps only reads the session registry and prunes dead entries; it never
invokes proot. Its companion kill was already exempt with exactly this
rationale, so requiring proot for ps was an inconsistency: on a system
without proot you could kill tracked sessions but not list them.

Author: sylirre

CLAUDE.md

@@ -151,8 +151,8 @@ numeric uid, or `user:group`.
 2. Root warn (non-fatal); nested-proot reject (reads
    `/proc/<pid>/status`, follows one TracerPid hop).
 3. proot probe; on Termux + TTY, offers `pkg install`. **`build`,
-   `push`, and `kill` are exempt** (`kill` only signals running
-   sessions); `build` runs its own gate via
+   `push`, `kill`, and `ps` are exempt** (`kill`/`ps` only signal or
+   read running sessions); `build` runs its own gate via
    `build_engine.needs_proot()` (True only with a RUN-family).
 4. Per-command `-h`/`--help`/`--usage` intercepted **before** argparse
    so missing positionals never produce errors instead of help. Unknown

proot_distro/cli.py

@@ -174,10 +174,11 @@ def _ensure_proot_available(first_canonical: str) -> None:
     `push` only reads from the local manifest/layer cache and uploads
     to a registry. `build` runs its own check after parsing the
     Dockerfile and refuses only when RUN (or ONBUILD RUN) is actually
-    present. `kill` is exempt too: it only signals already-running
-    sessions and never invokes proot.
+    present. `kill` and `ps` are exempt too: they only inspect and
+    signal already-running sessions via the session registry and never
+    invoke proot.
     """
-    if first_canonical in ("build", "push", "kill"):
+    if first_canonical in ("build", "push", "kill", "ps"):
         return
     ensure_proot_installed()