fix: skip read-only mounts in permission fixer and remove redundant ssh_keys manifest mount

- apply_mount_permissions now skips mounts with readonly=true to avoid
  'Read-only file system' warnings on post-start chown/chmod
- Removed the ssh_keys mount from the pi-agent manifest definition;
  instance-level SSH key mounting now handles this exclusively
- Added unit test for read-only mount skipping

Quality gates: pytest (15 passed)

apps/api/tests/unit/test_permission_fixer.py

@@ -64,6 +64,24 @@ class TestApplyMountPermissions:
             "find /home/user/.ssh -type f -exec chmod 0600" in file_mode_call[0][1][2]
         )
 
+    @patch("src.services.permission_fixer._run_in_container")
+    def test_skips_readonly_mount(self, mock_run) -> None:
+        mounts = [
+            {
+                "name": "ssh_keys",
+                "target": "/home/user/.ssh",
+                "readonly": True,
+                "mode": "0700",
+                "file_mode": "0600",
+            },
+        ]
+        results = apply_mount_permissions("abc123", mounts)
+
+        assert len(results) == 1
+        assert results[0]["mount_name"] == "ssh_keys"
+        assert results[0]["success"] is True
+        mock_run.assert_not_called()
+
     @patch("src.services.permission_fixer._run_in_container")
     def test_skips_mount_with_no_policy(self, mock_run) -> None:
         mounts = [