feat: add configurable project_serena_folder_location setting
Add a new `project_serena_folder_location` setting to `serena_config.yml` that allows configuring where the per-project `.serena` data folder (memories, caches) is stored, with `$projectDir` and `$projectFolderName` placeholder support (default: `$projectDir/.serena`, preserving current behavior).
Resolves#1123
Co-authored-by: Dominik Jain <dominik.jain@oraios-ai.de>
