Ensure root filesystem is always read-only inside sandbox

Whitelist mode's implicit bwrap root was a writable tmpfs, letting the
sandboxed process create files and directories anywhere not covered by
an explicit ro mount. This was not an issue in blacklist mode due to
--ro-bind / / covering that case.

This patch adds --remount-ro / before any other mount to make the base
layer read-only in both modes.

tests/integration.rs

@@ -420,6 +420,107 @@ fn blacklist_dev_input_hidden() {
     );
 }
 
+#[test]
+fn blacklist_root_is_readonly() {
+    let output = sandbox(&[])
+        .args([
+            "--",
+            "bash",
+            "-c",
+            "touch /rootfile 2>&1 && echo WRITABLE || echo READONLY; \
+             mkdir /newdir 2>&1 && echo MKDIR_OK || echo MKDIR_FAIL",
+        ])
+        .output()
+        .expect("agent-sandbox binary failed to execute");
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(
+        stdout.contains("READONLY"),
+        "expected root to be read-only in blacklist mode, got: {stdout}"
+    );
+    assert!(
+        stdout.contains("MKDIR_FAIL"),
+        "expected mkdir at root to fail in blacklist mode, got: {stdout}"
+    );
+}
+
+#[test]
+fn whitelist_root_is_readonly() {
+    let output = sandbox(&["--whitelist"])
+        .args([
+            "--",
+            "bash",
+            "-c",
+            "touch /rootfile 2>&1 && echo WRITABLE || echo READONLY; \
+             mkdir /newdir 2>&1 && echo MKDIR_OK || echo MKDIR_FAIL",
+        ])
+        .output()
+        .expect("agent-sandbox binary failed to execute");
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(
+        stdout.contains("READONLY"),
+        "expected root to be read-only in whitelist mode, got: {stdout}"
+    );
+    assert!(
+        stdout.contains("MKDIR_FAIL"),
+        "expected mkdir at root to fail in whitelist mode, got: {stdout}"
+    );
+}
+
+#[test]
+fn whitelist_mountpoint_parents_are_readonly() {
+    let output = sandbox(&["--whitelist"])
+        .args([
+            "--",
+            "bash",
+            "-c",
+            "echo pwned > /home/testfile 2>&1 && echo HOME_WRITABLE || echo HOME_READONLY; \
+             touch /etc/newfile 2>&1 && echo ETC_WRITABLE || echo ETC_READONLY; \
+             touch /var/newfile 2>&1 && echo VAR_WRITABLE || echo VAR_READONLY",
+        ])
+        .output()
+        .expect("agent-sandbox binary failed to execute");
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(
+        stdout.contains("HOME_READONLY"),
+        "expected /home to be read-only in whitelist mode, got: {stdout}"
+    );
+    assert!(
+        stdout.contains("ETC_READONLY"),
+        "expected /etc to be read-only in whitelist mode, got: {stdout}"
+    );
+    assert!(
+        stdout.contains("VAR_READONLY"),
+        "expected /var to be read-only in whitelist mode, got: {stdout}"
+    );
+}
+
+#[test]
+fn whitelist_tmp_still_writable() {
+    let output = sandbox(&["--whitelist"])
+        .args([
+            "--",
+            "bash",
+            "-c",
+            "touch /tmp/ok 2>&1 && echo TMP_WRITABLE || echo TMP_READONLY; \
+             touch /var/tmp/ok 2>&1 && echo VARTMP_WRITABLE || echo VARTMP_READONLY",
+        ])
+        .output()
+        .expect("agent-sandbox binary failed to execute");
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(
+        stdout.contains("TMP_WRITABLE"),
+        "expected /tmp to remain writable in whitelist mode, got: {stdout}"
+    );
+    assert!(
+        stdout.contains("VARTMP_WRITABLE"),
+        "expected /var/tmp to remain writable in whitelist mode, got: {stdout}"
+    );
+}
+
 #[test]
 fn rw_missing_path_errors() {
     let output = sandbox(&["--rw", "/nonexistent/xyz"])