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/workers/auto_update.rs

@@ -190,7 +190,7 @@ pub async fn ensure_root_agent(coord: &Arc<Coordinator>) -> Result<()> {
     // ensure_agent_runtime_dir needed here.
     let runtime = crate::paths::agent_runtime_dir(MANAGER_NAME);
     let hive = coord.hive_env();
-    let paths = Coordinator::agent_paths(MANAGER_NAME, runtime);
+    let paths = Coordinator::agent_paths(MANAGER_NAME, runtime)?;
     lifecycle::spawn(MANAGER_NAME, &hive, &paths).await?;
     seed_manager_tool_groups();
     if let Err(e) = coord.power.set(MANAGER_NAME, crate::power::Wanted::Up) {