feat(agent): add external MCP client support

Reuse OpenCode 2 HTTP/stdio clients with per-session read-only grants,
native approval and sanitized owner-scoped MCP status. Narrow the
environment inherited by trusted local MCP processes.

Validated in an isolated checkout with bun run check and 20 focused tests.

opencode.json

@@ -297,6 +297,11 @@
       "action": "geocode",
       "resource": "*",
       "effect": "allow"
+    },
+    {
+      "action": "mcp_*",
+      "resource": "*",
+      "effect": "ask"
     }
   ],
   "providers": {
@@ -373,5 +378,13 @@
         }
       }
     }
+  },
+  "mcp": {
+    "timeout": {
+      "startup": 10000,
+      "catalog": 10000,
+      "execution": 60000
+    },
+    "servers": {}
   }
 }