hive-c0re: fail on a malformed agent name and on an unreadable container list

An agent name that is not a valid Ident made `Coordinator::agent_paths`
panic. Job payloads carry names as plain strings (the swarm's published
wanted state is one source), and a panic inside a job-queue node never
reaches `complete_growing`, so the node's resources (the deploy
window included) were held until hive-c0re restarted. `agent_paths` now
returns an error; the job-queue nodes, the admin-socket spawn and
set-limits paths, the root-agent spawn and the dashboard set-limits
handler propagate it.

`lifecycle::list().await.unwrap_or_default()` turned a failed container
list into "no agents":
- meta-update cascade: the lock bump committed and zero rebuilds fanned
  out, reported as success. The cascade is now resolved before the lock
  bump and a list failure fails the node.
- dashboard update-all: queued nothing and returned 200 "ok". Now 500
  with the error.
- container rescan: every row was emitted as removed and the cache
  emptied. Now the last snapshot stands; `hivectl status` gets an error.
- dashboard journal: answered 404 "no managed container". Now 500.
- spawn/rebuild port-collision check: silently skipped. Now fails.
- startup migration: the per-agent phases ran over nothing, and phase 3
  handed an empty agent list to `meta::sync_agents`, which renders the
  meta flake with exactly the agents it is given. Both now log the list
  failure and skip.

The hive-jobq scheduler still leaks a node's resources on any executor
panic; that root is not addressed here.

Refs #4723

hive-c0re/src/lifecycle/tests.rs

@@ -377,3 +377,18 @@ fn failed_destroy_with_an_unreadable_list_fails() {
     .expect_err("an unreadable list must not pass for an absent container");
     assert!(format!("{err:#}").contains("cannot confirm"), "{err:#}");
 }
+
+#[test]
+fn agent_names_strips_the_container_prefix() {
+    let names = agent_names(Ok(vec![format!("{AGENT_PREFIX}iris"), "other".to_owned()]))
+        .expect("a readable list");
+    assert_eq!(names, vec!["iris"]);
+}
+
+/// A failed list is not an empty hive: the error reaches the caller.
+#[test]
+fn agent_names_propagates_an_unreadable_list() {
+    let err = agent_names(Err(anyhow::anyhow!("connect to hive-priv socket")))
+        .expect_err("an unreadable list must not read as zero agents");
+    assert!(format!("{err:#}").contains("hive-priv"), "{err:#}");
+}