fix(sandbox): emit warning when Landlock filesystem sandbox degrades silently (#599)

* fix(sandbox): emit warning when Landlock filesystem sandbox degrades silently

BestEffort Landlock previously swallowed failures at debug level, making
sandbox bypass invisible to operators at default log levels. Upgrade the
degradation log to warn with an actionable message pointing to the
hard_requirement setting. Add info-level startup log showing the
requested ABI and path counts so operators always know what Landlock
protections are active.

Closes #584

* fix(sandbox): revert unintended ABI bump from V2 to V5

Signed-off-by: John Myers <johntmyers@users.noreply.github.com>

---------

Signed-off-by: John Myers <johntmyers@users.noreply.github.com>
Co-authored-by: John Myers <johntmyers@users.noreply.github.com>

Author: johntmyers

crates/openshell-sandbox/src/sandbox/linux/landlock.rs

@@ -10,7 +10,7 @@ use landlock::{
 };
 use miette::{IntoDiagnostic, Result};
 use std::path::PathBuf;
-use tracing::debug;
+use tracing::{debug, info, warn};
 
 pub fn apply(policy: &SandboxPolicy, workdir: Option<&str>) -> Result<()> {
     let read_only = policy.filesystem.read_only.clone();
@@ -29,8 +29,16 @@ pub fn apply(policy: &SandboxPolicy, workdir: Option<&str>) -> Result<()> {
         return Ok(());
     }
 
+    let abi = ABI::V2;
+    info!(
+        abi = ?abi,
+        compatibility = ?policy.landlock.compatibility,
+        read_only_paths = read_only.len(),
+        read_write_paths = read_write.len(),
+        "Applying Landlock filesystem sandbox"
+    );
+
     let result: Result<()> = (|| {
-        let abi = ABI::V2;
         let access_all = AccessFs::from_all(abi);
         let access_read = AccessFs::from_read(abi);
 
@@ -71,7 +79,11 @@ pub fn apply(policy: &SandboxPolicy, workdir: Option<&str>) -> Result<()> {
             policy.landlock.compatibility,
             LandlockCompatibility::BestEffort
         ) {
-            debug!(error = %err, "Landlock unavailable, continuing without filesystem sandbox");
+            warn!(
+                error = %err,
+                "Landlock filesystem sandbox is UNAVAILABLE — running WITHOUT filesystem restrictions. \
+                 Set landlock.compatibility to 'hard_requirement' to make this a fatal error."
+            );
             return Ok(());
         }
         return Err(err);