WorktreeConfig.base_dir uses String where Option<String> is idiomatic

[wollax]

Problem

base_dir defaults to empty string via #[serde(default)], and resolve_worktree_dir checks .filter(|d| !d.is_empty()) to treat empty as absent. This "empty string means unset" pattern is fragile — a user setting base_dir = "" gets silent default behavior instead of a validation error.

Solution

Use Option<String> with #[serde(default, skip_serializing_if = "Option::is_none")] to make "unset" explicit.