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>

ue_mcp_server.py

@@ -207,15 +207,26 @@ def get_file(relative_path: str) -> str:
 def search_source(pattern: str, path_hint: str = "") -> str:
     """Grep UE source .h files for pattern. Requires $UE_ENGINE_ROOT.
     path_hint: optional subdirectory under Engine/Source/ (e.g. 'Runtime/Engine').
+    Searches both Engine/Source/ and Engine/Plugins/.
     Returns up to 40 matching lines with file:line context."""
-    root = _engine_root() / "Engine" / "Source"
-    if path_hint:
-        root = root / path_hint
-    result = subprocess.run(
-        ["grep", "-rn", "--include=*.h", "-m", "40", pattern, str(root)],
-        capture_output=True, text=True, timeout=15
+    engine = _engine_root()
+    candidates = [engine / "Source", engine / "Plugins"]
+    search_dirs = [d / path_hint if path_hint else d for d in candidates]
+    search_dirs = [d for d in search_dirs if d.exists()]
+    if not search_dirs:
+        return "(no matches)"
+    grep = subprocess.Popen(
+        ["grep", "-rn", "--include=*.h", pattern, *map(str, search_dirs)],
+        stdout=subprocess.PIPE, stderr=subprocess.DEVNULL, text=True
     )
-    return result.stdout or "(no matches)"
+    lines = []
+    for line in grep.stdout:
+        lines.append(line)
+        if len(lines) == 40:
+            grep.kill()
+            break
+    grep.wait()
+    return "".join(lines) or "(no matches)"
 
 
 if __name__ == "__main__":