feat: add file tools (read/write/edit/list_dir) for agent memory writes

Enable the agent to persist memories by adding 4 SPIFFS file tools
(read_file, write_file, edit_file, list_dir) with path validation,
and update the system prompt with memory guidelines pointing to
/spiffs/memory/MEMORY.md and daily notes.

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

main/CMakeLists.txt

@@ -16,6 +16,7 @@ idf_component_register(
         "tools/tool_registry.c"
         "tools/tool_web_search.c"
         "tools/tool_get_time.c"
+        "tools/tool_files.c"
     INCLUDE_DIRS
         "."
     REQUIRES