mars(kittentts): fix review round 3 — eval, delivery and stale-venv bugs

- pkgs.setpriv does not exist (mars failed to eval); setpriv is in util-linux.
- `sh` is not on a unit's PATH; call ${pkgs.runtimeShell} directly.
- Drop `env HOME="$hermesHome"`: unset shell var under `set -u`, and unneeded.
- Staging dir 0700 -> 0755: the uid-986 delivery step could not read it.
- Rebuild the staging venv whenever the stamp misses, else an input bump
  delivered the old venv under the new stamp.
- Add diffutils: `cmp` in the fast path was missing, so every boot re-delivered.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

hosts/mars/hermes-agent.nix

@@ -492,7 +492,8 @@ in
   # is ever actually used.
   #
   # Provisioner invariants (shaped by the Mnemosyne + KittenTTS review rounds):
-  #   - Build in a ROOT-OWNED staging dir (/var/lib/hermes-kittentts, 0700):
+  #   - Build in a ROOT-OWNED staging dir (/var/lib/hermes-kittentts, 0755 so
+  #     the uid-986 delivery step can read it; nothing in it is secret):
   #     stamp, staging venv, uv cache and staging model copies all live there.
   #     Root never reads or executes anything the container can write — the
   #     container cannot symlink-takeover the stamp (review #1) or plant a
@@ -533,7 +534,8 @@ in
       "hermes-agent-prepare-dirs.service"
     ];
     requires = [ "hermes-agent-prepare-dirs.service" ];
-    path = [ pkgs.uv pkgs.coreutils pkgs.setpriv ];
+    # diffutils: `cmp` in the fast path; missing, it silently re-delivers every boot.
+    path = [ pkgs.uv pkgs.coreutils pkgs.diffutils pkgs.util-linux ];
     serviceConfig = {
       Type = "oneshot";
       TimeoutStartSec = 600;
@@ -556,7 +558,7 @@ in
       # any uid-986-writable path while running as root.
       stageDir=/var/lib/hermes-kittentts
       # Root-side staging of the DELIVERED TREE (venv + full HF hub layout
-      # including refs/main) — everything root writes lives here, 0700.
+      # including refs/main) — everything root writes lives here.
       # review2 #4: root never writes into hermesHome; delivery happens as
       # the container uid via setpriv, copying from these root-owned sources.
       stageVenv=$stageDir/venv
@@ -595,14 +597,17 @@ in
       # running as root; symlink-takeover of stamps/refs/install targets is
       # structurally impossible. ----
       mkdir -p "$stageDir" "$stageSnap" "$stageVenv" "$stageHub"
-      chmod 0700 "$stageDir"
+      # 0755: the setpriv'd uid-986 delivery below must be able to read it.
+      chmod 0755 "$stageDir"
       install -m 0444 ${kittenttsModelConfig} "$stageSnap/config.json"
       install -m 0444 ${kittenttsModelOnnx}   "$stageSnap/kitten_tts_mini_v0_8.onnx"
       install -m 0444 ${kittenttsModelVoices} "$stageSnap/voices.npz"
 
       # Skip the venv REBUILD when staging is still valid (review2 minor:
-      # damaged delivery should be a copy, not a rebuild).
-      if ! [ -x "$stageVenv/bin/python" ] \
+      # damaged delivery should be a copy, not a rebuild) — but only for the
+      # CURRENT inputs, else a bump delivers the old venv under a new stamp.
+      if [ "$(cat "$stampFile" 2>/dev/null)" != "$inputHash" ] \
+         || ! [ -x "$stageVenv/bin/python" ] \
          || ! [ -f "$stageVenv/lib/python3.13/site-packages/kitten_tts_stub.py" ]; then
         rm -rf "$stageVenv"
         UV_CACHE_DIR=$stageDir/uv-cache \
@@ -652,7 +657,7 @@ in
       # fresh staging tree in. cp-as-986 also fixes review2 #2: rm+cp in one
       # step, no mv -Tf rename-replace on a non-empty directory.
       setpriv --reuid=${hermesUid} --regid=${hermesGid} --clear-groups \
-        env HOME="$hermesHome" sh -c '
+        ${pkgs.runtimeShell} -c '
           set -eu
           rm -rf "$1" "$2"
           cp -a "$3" "$1"