chore(tooling): wire CodeGraph MCP server + agent guidance

Add the codegraph stdio MCP server to .mcp.json and the CodeGraph usage
guidance block to AGENTS.md, so coding agents working in this repo can use the
codegraph_* structural-search tools.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.mcp.json

@@ -10,13 +10,19 @@
     "sequential-thinking": {
       "type": "stdio",
       "command": "npx",
-      "args": ["-y", "@modelcontextprotocol/server-sequential-thinking"],
+      "args": [
+        "-y",
+        "@modelcontextprotocol/server-sequential-thinking"
+      ],
       "env": {}
     },
     "github": {
       "type": "stdio",
       "command": "npx",
-      "args": ["-y", "@modelcontextprotocol/server-github"],
+      "args": [
+        "-y",
+        "@modelcontextprotocol/server-github"
+      ],
       "env": {
         "GITHUB_PERSONAL_ACCESS_TOKEN": "${GITHUB_PERSONAL_ACCESS_TOKEN}"
       }
@@ -24,14 +30,27 @@
     "fetch": {
       "type": "stdio",
       "command": "uvx",
-      "args": ["mcp-server-fetch"],
+      "args": [
+        "mcp-server-fetch"
+      ],
       "env": {}
     },
     "playwright": {
       "type": "stdio",
       "command": "npx",
-      "args": ["-y", "@playwright/mcp@latest"],
+      "args": [
+        "-y",
+        "@playwright/mcp@latest"
+      ],
       "env": {}
+    },
+    "codegraph": {
+      "type": "stdio",
+      "command": "codegraph",
+      "args": [
+        "serve",
+        "--mcp"
+      ]
     }
   }
 }