Paths and XDG mapping

Path resolution lives in crates/hometree-core/src/paths.rs. All paths are derived from XDG locations unless overrides are provided.

Defaults (XDG-aware)

Helper	Default location	Notes
`home_dir()`	`$HOME`	Base for relative paths in the config.
`config_dir()`	`$XDG_CONFIG_HOME/hometree` (fallback `~/.config/hometree`)	Directory that stores `config.toml`.
`data_dir()`	`$XDG_DATA_HOME/hometree` (fallback `~/.local/share/hometree`)	Holds the bare git repo.
`state_dir()`	`$XDG_STATE_HOME/hometree` (fallback to `$XDG_DATA_HOME/hometree`)	Stores deploy/snapshot metadata.
`cache_dir()`	`$XDG_CACHE_HOME/hometree` (fallback `~/.cache/hometree`)	Scratch/cache space.
`config_home_dir()`	parent of `config_dir()`	Equivalent to `$XDG_CONFIG_HOME`.
`runtime_dir()`	`$HOMETREE_RUNTIME_DIR/hometree` or `$XDG_RUNTIME_DIR/hometree`	IPC socket and runtime files for the daemon.

Derived files:

Helper	Path	Notes
`config_file()`	`config_dir()/config.toml`	Main configuration file.
`repo_dir()`	`data_dir()/repo.git`	Bare git repository used by hometree.

--home-root <path> sets home_dir to <path> and, if no --xdg-root is provided, maps XDG dirs under that home (e.g. <home>/.config/hometree).
--xdg-root <path> forces XDG mapping to <path>/config|data|state|cache/ with the hometree suffix, regardless of home_root.
HOMETREE_RUNTIME_DIR overrides the runtime dir; otherwise XDG_RUNTIME_DIR is used.
If neither flag is provided, standard XDG environment variables are honored via directories::BaseDirs.

Home override only: --home-root /tmp/home -> config at /tmp/home/.config/hometree/config.toml, repo at /tmp/home/.local/share/hometree/repo.git.
Home + XDG override: --home-root /tmp/home --xdg-root /tmp/xdg -> config at /tmp/xdg/config/hometree/config.toml, repo at /tmp/xdg/data/hometree/repo.git.