dashboard: K3PT ST4T3 section + agent links open in new tab

new section between containers and questions: lists every name with a
state dir under /var/lib/hyperhive/agents/ that doesn't correspond to
a live container. shows state size + last-modified age + whether
claude creds are kept. two actions per row:

- R3V1V3 — queues a spawn approval with the same name (operator
  approves to recreate; spawn flow reuses prior config + claude
  creds, no re-login needed)
- PURG3 — wipes the agent's state + applied dirs (post /purge-tombstone/
  endpoint; refuses if a live container with that name still exists)

dashboard also opens agent links in new tabs now (target=_blank +
rel=noopener) so the operator's overview tab stays put when they
dive into an agent.

hive-c0re/src/coordinator.rs

@@ -204,4 +204,23 @@ impl Coordinator {
     pub fn agent_applied_dir(name: &str) -> PathBuf {
         PathBuf::from(format!("{APPLIED_STATE_ROOT}/{name}"))
     }
+
+    /// Enumerate names that have a persistent state dir under
+    /// `/var/lib/hyperhive/agents/` (i.e. config / claude creds /
+    /// notes survive). Includes both currently-existing containers and
+    /// destroyed-but-kept tombstones; callers filter the latter by
+    /// subtracting `lifecycle::list()`.
+    #[must_use]
+    pub fn kept_state_names() -> Vec<String> {
+        let Ok(rd) = std::fs::read_dir(AGENT_STATE_ROOT) else {
+            return Vec::new();
+        };
+        let mut out: Vec<String> = rd
+            .flatten()
+            .filter(|e| e.file_type().is_ok_and(|t| t.is_dir()))
+            .filter_map(|e| e.file_name().into_string().ok())
+            .collect();
+        out.sort();
+        out
+    }
 }