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/migrate.rs

@@ -63,7 +63,13 @@ pub async fn run(coord: &Arc<Coordinator>) -> Result<()> {
             Err(e) => tracing::warn!(error = ?e, "clear stale meta lock failed"),
         }
     }
-    let names = enumerate_agents().await;
+    let names = match enumerate_agents().await {
+        Ok(names) => names,
+        Err(e) => {
+            tracing::warn!(error = ?e, "migration: container list failed; skipping per-agent phases");
+            Vec::new()
+        }
+    };
     tracing::info!(count = names.len(), "migration: scanning");
 
     // Phase 0: move harness-owned files out of state/ into harness/.
@@ -94,9 +100,15 @@ pub async fn run(coord: &Arc<Coordinator>) -> Result<()> {
 
     // Phase 3: meta repo.
     tracing::debug!("migration: phase 3 (meta sync_agents)");
-    let agents = lifecycle::agents_for_meta_listing()
-        .await
-        .unwrap_or_default();
+    // `sync_agents` renders exactly the agents it is given, so an empty
+    // stand-in for an unreadable list would drop every agent from the flake.
+    let agents = match lifecycle::agents_for_meta_listing().await {
+        Ok(agents) => agents,
+        Err(e) => {
+            tracing::warn!(error = ?e, "migration: container list failed; skipping meta sync_agents");
+            return Ok(());
+        }
+    };
     match tokio::time::timeout(GIT_TIMEOUT, meta::sync_agents(&coord.hive_env(), &agents)).await {
         Ok(Err(e)) => tracing::warn!(error = ?e, "migration: meta sync_agents failed"),
         Err(_) => {
@@ -139,9 +151,9 @@ fn migrate_harness_files(name: &hive_types::Ident) {
     }
 }
 
-async fn enumerate_agents() -> Vec<hive_types::Ident> {
-    let containers = lifecycle::list().await.unwrap_or_default();
-    containers
+async fn enumerate_agents() -> Result<Vec<hive_types::Ident>> {
+    let containers = lifecycle::list().await?;
+    Ok(containers
         .into_iter()
         .filter_map(|c| {
             let name = if c == MANAGER_CONTAINER {
@@ -151,7 +163,7 @@ async fn enumerate_agents() -> Vec<hive_types::Ident> {
             };
             hive_types::Ident::parse(name).ok()
         })
-        .collect()
+        .collect())
 }
 
 async fn migrate_applied_repo(name: &str) -> Result<()> {