fix: report a clear cli error when no herdr server is running (#1963)

* fix: report a clear cli error when no herdr server is running

socket cli commands surfaced a raw io::Error debug string
(`Error: Os { code: 2, ... }`) when nothing was listening on the
api socket, which read like a bad --cwd path. map dead-socket
connect failures to a `server_not_running` json error carrying the
resolved socket path, printed once at the edge that surfaces the
error; recovering callers (plugin offline registry fallback, agent
start polling) recognize the marker and keep their existing
behavior.

refs #1941

* fix: map dead-socket errors on the unchecked cli request path

refs #1941

* fix: make server-not-running guidance session-aware

refs #1941

src/cli/plugin.rs

@@ -1618,14 +1618,19 @@ fn current_unix_ms() -> u64 {
 }
 
 fn is_connection_error(err: &std::io::Error) -> bool {
-    matches!(
-        err.kind(),
-        std::io::ErrorKind::NotFound
-            | std::io::ErrorKind::ConnectionRefused
-            | std::io::ErrorKind::ConnectionAborted
-            | std::io::ErrorKind::ConnectionReset
-            | std::io::ErrorKind::BrokenPipe
-    )
+    // A `server_not_running` marker is a connect failure for recovery purposes:
+    // treating it as a connection error lets plugin commands fall back to the
+    // offline registry. The marker carries (but does not print) a friendly
+    // response, so recovering here prints nothing.
+    super::server_not_running_was_reported(err)
+        || matches!(
+            err.kind(),
+            std::io::ErrorKind::NotFound
+                | std::io::ErrorKind::ConnectionRefused
+                | std::io::ErrorKind::ConnectionAborted
+                | std::io::ErrorKind::ConnectionReset
+                | std::io::ErrorKind::BrokenPipe
+        )
 }
 
 fn print_plugin_response(method: Method) -> std::io::Result<i32> {