fix(apparmor): correct the sudo profile comment's conclusion

The abort needs havePrivateMountNs == false AND /nix/store still ro:
remountReadOnlyWritable() returns early while the store is writable. Nix
<= 2.34 left it rw until reboot; 2.35 remounts in a private namespace, so
only a freshly booted host hits it. The repro command also uses an open-only
exec 7<..., since cat fails on the read even when the open succeeds.

While here, trim the comments and drop the disabled default_deny/nix
policies plus the explicit killUnconfinedConfinables=false (nixpkgs
default). 82 -> 34 lines.

hardening/apparmor/default.nix

@@ -1,82 +1,37 @@
 {
-  config,
   pkgs,
   ...
 }:
 {
-  # AppArmor: activate the LSM + load policies. Roll out in complain mode first
-  # (log violations, don't block) so nothing can break; promote profiles to
-  # "enforce" individually once stable. Complain mode is not absolute: opens of
-  # disconnected paths (/proc/self/ns/*, /proc/self/root, ...) are denied even in
-  # complain mode unless the profile sets flags=(attach_disconnected); see the
-  # sudo policy below.
+  # AppArmor in complain mode: log violations instead of blocking.
   services.dbus.apparmor = "enabled";
 
   security.apparmor = {
     enable = true;
 
-    # Do not SIGTERM running unconfined-but-confinable processes yet.
-    # Safe to flip to true later now that no global default-deny profile is active.
-    killUnconfinedConfinables = false;
-
-    # Packages contributing to AppArmor's include path (abstractions).
-    # NOTE: these abstractions are FHS-oriented and reference FHS paths. nixpkgs does NOT
-    # provide a broad FHS->store alias layer (only /run/current-system, /usr/bin/env and
-    # /bin/sh are created), so some referenced paths do not resolve on NixOS and the
-    # resulting profiles are incomplete. They are safe only because they run in complain
-    # mode (log-only, no blocking); see hardening/README.md.
+    # FHS-oriented abstractions, incomplete on NixOS; only safe in complain mode.
     packages = [ pkgs.apparmor-profiles ];
 
-    policies = {
-      # Global default-deny scaffold. DANGEROUS to enable: `/**` matches every binary and
-      # the empty block allows nothing. Keep disabled until per-app profiles exist.
-      "default_deny" = {
-        state = "disable";
-        profile = "profile default_deny /** { }";
-      };
+    policies.sudo = {
+      state = "complain";
+      profile = ''
+        abi <abi/4.0>,
+        include <tunables/global>
 
-      # Confine sudo; complain mode so a missing rule logs instead of blocking.
-      "sudo" = {
-        state = "complain";
-        profile = ''
-          abi <abi/4.0>,
-          include <tunables/global>
-
-          # NOTE: exec must use the `ix` (inherit) modifier here: abstractions/base
-          # pulls in nested rules like `rix` on specific binaries (via xdg-open etc.),
-          # and AppArmor 5.0's parser rejects merged rules with conflicting exec
-          # modifiers ("profile has merged rule with conflicting x modifiers") when a
-          # broad rule uses Ux over those paths.
-          #
-          # NOTE: `attach_disconnected` is required. Magic links such as
-          # /proc/self/ns/mnt and /proc/self/root have no connected path, so AppArmor
-          # cannot match them against any rule and denies the open with EACCES even in
-          # complain mode. Nix 2.35 (nix-env --set as root) opens /proc/self/ns/mnt in
-          # its saveMountNamespace(), and once that fails the store is never remounted
-          # writable and nixos-rebuild dies with:
-          #   cannot remount "/nix/store" writable: not in a private mount namespace
-          # Reproduce the underlying denial with:
-          #   aa-exec -p ${pkgs.sudo}/bin/sudo -- cat /proc/self/ns/mnt
-          #
-          # Rolling this out to a machine that already has the old (non-attach)
-          # profile loaded needs one deploy where the *new* system's Nix does not
-          # run the profile switch, e.g.
-          #   nixos-rebuild boot --sudo --no-reexec --flake .#<host> --accept-flake-config
-          # (with --no-reexec the current system's Nix, 2.34, runs nix-env --set and
-          # avoids the broken profile). After that generation is active, plain
-          # `just niri` works again and the Nix version is never rolled back.
-          profile ${pkgs.sudo}/bin/sudo flags=(attach_disconnected) {
-            include <abstractions/base>
-            file /** rwlkix,
-          }
-        '';
-      };
-
-      # nix runs unconfined (no restriction); inert, kept disabled.
-      "nix" = {
-        state = "disable";
-        profile = "profile ${config.nix.package}/bin/nix { unconfined, }";
-      };
+        # Confine sudo (complain). `attach_disconnected` lets Nix 2.35's
+        # nixos-rebuild set the profile; `ix` keeps the parser happy.
+        #
+        # /proc/self/ns/mnt has no connected path, so AppArmor denies the open even in
+        # complain mode; Nix 2.35's root nix-env opens it in saveMountNamespace(), and
+        # nixos-rebuild aborts only while /nix/store is ro (<= 2.34 left it rw until
+        # reboot, 2.35 remounts in a namespace).
+        # abstractions/base has nested rix rules, and Ux makes AppArmor 5.0 reject
+        # the merged rules.
+        profile ${pkgs.sudo}/bin/sudo flags=(attach_disconnected) {
+          include <abstractions/base>
+          file /** rwlkix,
+        }
+      '';
     };
   };
 }