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/container_view.rs

@@ -152,8 +152,12 @@ fn agent_url_for_domain(domain: &str, name: &str) -> String {
 /// fallback onto the hive-wide `agent_cpu_quota` / `agent_memory_max`,
 /// and those live on [`crate::coordinator::HiveEnv`], not on disk. Both callers already
 /// hold a `Coordinator`, so this is a parameter rather than a global.
-pub async fn build_all(hive: &crate::coordinator::HiveEnv) -> Vec<ContainerView> {
-    let raw = lifecycle::list().await.unwrap_or_default();
+///
+/// # Errors
+///
+/// The container list could not be read.
+pub async fn build_all(hive: &crate::coordinator::HiveEnv) -> anyhow::Result<Vec<ContainerView>> {
+    let raw = lifecycle::list().await?;
     let locked = read_meta_locked_revs();
     // Read once per scan rather than re-read for every container on every
     // SSE scan. An unreadable file shows every row at the hive defaults;
@@ -254,7 +258,7 @@ pub async fn build_all(hive: &crate::coordinator::HiveEnv) -> Vec<ContainerView>
             memory_max,
         });
     }
-    out
+    Ok(out)
 }
 
 /// Host-side mirror of `hive_agent::login::has_session`. Returns true