feat(sandbox): Landlock no-op sous Posture::Allow (allow-by-default)

Un allowlist Landlock ne peut que retirer des accès (deny-by-default). Sous
une fallback Posture::Allow (allow-by-default), enforcer les grants
verrouillerait tout hors du project root — binaire CLI, libs et ~/.<cli> —
provoquant le 'failed to open terminal'. On n'enforce donc rien sous Allow ;
seuls les Deny explicites font foi (advisory via la projection LP3).
Couvre permissions.json persisté avec fallback allow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/src/sandbox/landlock.rs

@@ -90,6 +90,20 @@ impl SandboxEnforcer for LandlockSandbox {
     fn enforce(&self, plan: &SandboxPlan) -> Result<SandboxStatus, SandboxError> {
         let abi = TARGET_ABI;
 
+        // Landlock is an **additive allowlist** — it can only *remove* access, i.e.
+        // it is deny-by-default. A `Posture::Allow` fallback means the opposite,
+        // **allow-by-default**: paths matched by no grant must stay reachable. An
+        // allowlist cannot express that (it would lock everything down to the granted
+        // roots, closing the CLI binary, its libs and `~/.<cli>` home — the very
+        // "failed to open terminal" symptom). So under an Allow fallback we enforce
+        // nothing: the policy's only teeth are explicit Denies, which are deny-islands
+        // under an allow-all that an additive sandbox cannot carve out — they remain
+        // advisory via the LP3 projection. This is a property of the allowlist
+        // mechanism, hence it lives in the adapter, not the pure plan.
+        if plan.default_posture == Posture::Allow {
+            return Ok(SandboxStatus::Enforced);
+        }
+
         // Handle a right class only if the plan posed a grant of that class, so an
         // unposed class stays globally unrestricted (per-access-class autonomy).
         let mut handled = BitFlags::<AccessFs>::empty();
@@ -283,6 +297,59 @@ mod tests {
         assert_eq!(status, SandboxStatus::Enforced);
     }
 
+    /// **Allow fallback ⇒ no OS restriction even with grants posed.** Setting the
+    /// policy to allow-by-default (and, in the UI, every option to Allow) emits RO|RW
+    /// grants scoped to the project root. An allowlist sandbox would then close every
+    /// read/write OUTSIDE that root — locking out the CLI binary, its libs and home,
+    /// the exact "failed to open terminal" bug. Under `Posture::Allow` the adapter must
+    /// instead enforce nothing: from the (would-be) sandboxed thread, a read AND a
+    /// write OUTSIDE the only grant must both still succeed.
+    #[test]
+    fn allow_fallback_does_not_restrict_even_with_grants() {
+        let granted = fresh_dir("allow-granted");
+        let outside = fresh_dir("allow-outside");
+        std::fs::write(outside.join("pre.txt"), b"pre").unwrap();
+        let outside_t = outside.clone();
+
+        let plan = SandboxPlan {
+            allowed: vec![PathGrant {
+                abs_root: granted.to_string_lossy().into_owned(),
+                access: PathAccess::RO.union(PathAccess::RW),
+            }],
+            default_posture: Posture::Allow,
+        };
+
+        let (status, read_outside, write_outside) = std::thread::spawn(move || {
+            let status = LandlockSandbox::new()
+                .enforce(&plan)
+                .expect("an Allow fallback never fails closed");
+            let read = std::fs::read(outside_t.join("pre.txt"));
+            let write =
+                std::fs::write(outside_t.join("new.txt"), b"x").map_err(|e| e.kind());
+            (status, read, write)
+        })
+        .join()
+        .unwrap();
+
+        assert_eq!(
+            status,
+            SandboxStatus::Enforced,
+            "Allow fallback is a no-op enforcement, not an error or a lock-down"
+        );
+        assert!(
+            read_outside.is_ok(),
+            "Allow fallback must keep reads outside the grant open, got {read_outside:?}"
+        );
+        assert_eq!(
+            write_outside,
+            Ok(()),
+            "Allow fallback must keep writes outside the grant open"
+        );
+
+        let _ = std::fs::remove_dir_all(&granted);
+        let _ = std::fs::remove_dir_all(&outside);
+    }
+
     /// **CRITICAL SAFETY PROPERTY — IdeA is never sandboxed.** The adapter restricts
     /// the *current thread*; the launch path runs it on a throwaway thread. This test
     /// proves the Landlock domain is confined to that throwaway thread and does **not**