feat(developer): replace agents iframe with informational landing and live agent table

LangGraph is API-only with no web UI, so the ToolEmbed iframe pattern
doesn't work. Replace it with an informational landing page featuring a
hero section, connection status indicator, quick-start card grid linking
to docs/GitHub/examples, and a live table of registered agents fetched
from the LangGraph POST /assistants/search endpoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/i18n/en.json

@@ -98,7 +98,27 @@
     "tokens_used": "Tokens Used",
     "error_rate": "Error Rate",
     "not_configured": "Not Configured",
-    "open_new_tab": "Open in New Tab"
+    "open_new_tab": "Open in New Tab",
+    "agents_status_connected": "Connected",
+    "agents_status_not_connected": "Not Connected",
+    "agents_config_hint": "Set LANGGRAPH_URL in .env to connect",
+    "agents_quick_start": "Quick Start",
+    "agents_docs": "Documentation",
+    "agents_docs_desc": "Official LangGraph documentation and API guides.",
+    "agents_getting_started": "Getting Started",
+    "agents_getting_started_desc": "Step-by-step tutorial to build your first agent.",
+    "agents_github": "GitHub",
+    "agents_github_desc": "Source code, issues, and community contributions.",
+    "agents_examples": "Examples",
+    "agents_examples_desc": "Ready-to-use templates and example agent projects.",
+    "agents_api_ref": "API Reference",
+    "agents_api_ref_desc": "Local Swagger docs for your LangGraph instance.",
+    "agents_running_title": "Running Agents",
+    "agents_none": "No agents registered. Deploy an assistant to LangGraph to see it here.",
+    "agents_col_name": "Name",
+    "agents_col_id": "ID",
+    "agents_col_description": "Description",
+    "agents_col_status": "Status"
   },
   "org": {
     "title": "Organization",