hive-c0re: an unreadable capabilities file denies ManageRootAgent mounts

set_nspawn_flags propagated has_cap's error, so one corrupt
capabilities.json failed every agent's Start, spawn and Swap. It now
goes through holds_manage_root_agent, which logs the error (agent and
file) and treats the capability as absent: the agent starts without the
cross-agent, /applied and /meta mounts. caps_for/has_cap take the file
path so that seam is testable against a tempfile.

- meta.rs: a comment at the render_flake reads records why they
  propagate (an empty tool-groups map renders toolGroups = null, i.e.
  AGENT_DEFAULT, which fails open for narrower explicit entries).
- capabilities::read doc: states when set_caps/remove_agent rewrite
  the file instead of saying remove_agent repairs it.
- set/remove corrupt-file tests assert ErrorKind::InvalidData.

hive-c0re/src/agent_config/tool_groups.rs

@@ -127,15 +127,21 @@ mod tests {
     #[test]
     fn set_groups_leaves_a_corrupt_file_untouched() {
         let (_dir, path) = corrupt_file();
-        set_groups_at(&path, "ruth", &["messaging".to_owned()])
+        let err = set_groups_at(&path, "ruth", &["messaging".to_owned()])
             .expect_err("a corrupt file must not be overwritten");
+        let kind = err
+            .downcast_ref::<std::io::Error>()
+            .map(std::io::Error::kind);
+        assert_eq!(kind, Some(std::io::ErrorKind::InvalidData));
         assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
     }
 
     #[test]
     fn remove_agent_leaves_a_corrupt_file_untouched() {
         let (_dir, path) = corrupt_file();
-        remove_agent_at(&path, "alice").expect_err("a corrupt file must not be overwritten");
+        let err =
+            remove_agent_at(&path, "alice").expect_err("a corrupt file must not be overwritten");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
         assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
     }