enhance(config, cli): resolve --cfg paths across multiple roots

[JeanMertz]

Previously, --cfg path resolution only searched for files relative to the workspace root. This made it impossible to define personal configuration fragments that apply across all workspaces (in the user-global root) or fragments scoped to a single workspace but kept private (in the user-workspace root).

This commit implements RFD 035, extending --cfg resolution to search across three config roots in precedence order:

1. User-global: $XDG_CONFIG_HOME/jp/config/
2. Workspace: <workspace_root>/
3. User-workspace: $XDG_DATA_HOME/jp/workspace/<id>/config/

When a --cfg path is provided, JP now searches each root independently. If matching files are found in multiple roots, they are all loaded and merged in precedence order (highest precedence wins). Within a single root, the existing "first match wins" behavior for entries in config_load_paths is preserved.

Additionally, the MissingConfigFile error now includes a list of all locations that were searched, improving the debugging experience when a config fragment cannot be found.

For example, running jp --cfg skill/web will now search for and merge matches from your global config, the current workspace, and your workspace-specific private config.