feat(0049k): graph_explorer wiring + close issue 0049

- cpp/CMakeLists.txt: register projects/osint_graph/apps/graph_explorer/
  via add_subdirectory pattern (igual que registry_dashboard).
- dev/feature_flags.json: osint_graph_v1 = true (enabled_at 2026-04-30).
- dev/issues/{0049,0049k} → dev/issues/completed/. README index actualizado.

La app vive en su sub-repo dataforge/graph_explorer (push hecho al cerrar).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/CMakeLists.txt

@@ -215,6 +215,12 @@ if(EXISTS ${_DASH_DIR}/CMakeLists.txt)
     add_subdirectory(${_DASH_DIR} ${CMAKE_BINARY_DIR}/apps/registry_dashboard)
 endif()
 
+# --- Graph Explorer (lives in projects/osint_graph/apps/) ---
+set(_GE_DIR ${CMAKE_SOURCE_DIR}/../projects/osint_graph/apps/graph_explorer)
+if(EXISTS ${_GE_DIR}/CMakeLists.txt)
+    add_subdirectory(${_GE_DIR} ${CMAKE_BINARY_DIR}/apps/graph_explorer)
+endif()
+
 # --- Tests (Catch2 amalgamated, ctest-driven) ---
 option(BUILD_TESTING "Build C++ tests" ON)
 if(BUILD_TESTING)