fix(brain): exclude host-policy actions on Ubuntu Core

crates/sysknife-brain/src/planning_tools/propose_plan.rs

@@ -479,6 +479,11 @@ reports live interface state"),
 fn available_on(action: &str, hint: Option<&sysknife_types::DistroHint>) -> bool {
     let family = hint.map(|hint| hint.family);
     if let Some(hint) = hint {
+        // Ubuntu Core is immutable and is not an eligible Debian host. Do not
+        // offer apt (or other host-policy actions) merely from its family tag.
+        if hint.id == "ubuntu-core" {
+            return !action_requires_supported_host(action);
+        }
         if hint.id != "ubuntu" && UBUNTU_ONLY_ACTIONS.contains(&action) {
             return false;
         }
@@ -735,6 +740,20 @@ mod tests {
 
     #[test]
     fn debian_host_does_not_inherit_ubuntu_tools_or_preferences() {
+        let core = sysknife_types::DistroHint {
+            id: "ubuntu-core".into(),
+            family: DISTRO_FAMILY_DEBIAN,
+            version: Some("24".into()),
+        };
+        let core_def = propose_plan_tool_def(Some(&core));
+        let core_actions = offered_actions(&core_def);
+        for (action, _) in KNOWN_ACTIONS {
+            assert_eq!(
+                core_actions.contains(&action.to_string()),
+                !action_requires_supported_host(action),
+                "Ubuntu Core: {action}"
+            );
+        }
         let hint = sysknife_types::DistroHint {
             id: "debian".into(),
             family: DISTRO_FAMILY_DEBIAN,

crates/sysknife-daemon/src/dispatcher.rs

@@ -5079,7 +5079,8 @@ mod tests {
         // `AptUpdate` is RiskLevel::Low, so min_role_for_action puts it at
         // Observer — yet it runs `sudo apt-get update`. Any exemption keyed on
         // the RBAC role therefore lets a privileged mutation through, which is
-        // why the platform gate does not exempt reads at all.
+        // why the platform gate does not exempt hard-fenced reads. Portable
+        // Observer reads still return early without requiring distro detection.
         let dir = tempdir().unwrap();
         let mut state = test_state(&dir);
         state.host_distro = Some(sysknife_core::distro::DistroId::Debian { version: Some(11) });

docs/action-compatibility.md

@@ -27,6 +27,11 @@ longer require distro detection after leaving the hard lists. This widens
 read-only inspection; mutations still require an eligible host, and the
 Low-risk mutating `AptUpdate` remains hard-fenced.
 
+Ubuntu Core is not an eligible host. Its Debian family tag does not make apt
+available: the planner limits its catalogue to actions outside the shared
+host-policy predicate, as it does for an unknown family. This does not claim
+Ubuntu Core execution support.
+
 The MCP surface uses the same routing checks and withholds hard-restricted actions when
 detection fails. The planner receives an explicit distribution ID alongside the
 family; display text never grants Ubuntu capabilities.