Fix search_source: pass env vars via mcp.json interpolation, search Plugins/, cap at 40 total lines

- .mcp.json: pass UE_ENGINE_ROOT and UE_DOCS_PATH via ${VAR} interpolation so the
  MCP server subprocess inherits them without hardcoding paths
- search_source: search both Engine/Source/ and Engine/Plugins/ (path_hint applied
  under both); switch from -m 40 (per-file cap) to streaming Popen with a true
  40-line total cap; fix UE_ENGINE_ROOT to point at Engine/ not the repo root
- Update README and CLAUDE.md to reflect the above

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

.mcp.json

@@ -2,7 +2,11 @@
   "mcpServers": {
     "ue-docs": {
       "command": "python",
-      "args": ["ue_mcp_server.py"]
+      "args": ["ue_mcp_server.py"],
+      "env": {
+        "UE_ENGINE_ROOT": "${UE_ENGINE_ROOT}",
+        "UE_DOCS_PATH": "${UE_DOCS_PATH}"
+      }
     }
   }
 }