fix(sessions): invalidate session_index cache in session_delete

session_delete cleared the per-session state (session_hist_ and
session_node_) but not the shared session_index cache. The next call
to session_list() hit the fast path (state_get("session_index")) and
returned the deleted session until the daemon restarted.

session_update_patch already called state_set("session_index","") to
force a re-fetch from Engram; session_delete now does the same.

Add tests/test_sessions.el covering:
- session_title_from_message (pure function, all edge cases)
- session_make_content (JSON structure and required session:meta marker)
- DELETE cache invalidation: session_index cleared, fast path disabled
- PATCH cache invalidation: stale title/folder not returned via fast path
- GET /api/sessions: session_list() fast path returns session_index
  (confirms removal of the stale route_sessions() engram stub)

sessions.el

@@ -217,9 +217,11 @@ fn session_delete(session_id: String) -> String {
         } else { deleted_msgs }
         let j = j + 1
     }
-    // Clear state
+    // Clear state — invalidate all per-session and index caches so session_list()
+    // does not return this deleted session via the fast path on the next call.
     state_set("session_hist_" + session_id, "")
     state_set("session_node_" + session_id, "")
+    state_set("session_index", "")
     return "{\"ok\":true,\"session_id\":\"" + session_id + "\""
         + ",\"deleted_meta\":" + int_to_str(deleted_meta)
         + ",\"deleted_msgs\":" + int_to_str(deleted_msgs) + "}"