refactor: remove rootDir param, auto-detect .git as boundary

Walk up from task dir to find .git instead of threading rootDir through
Globs, checkers, and itemsFromFor. Gitignore rules are discarded if no
.git is found, matching ripgrep's require_git behavior. This keeps the
Globs signature clean and makes the future .taskignore integration
straightforward (loaded at setup like .taskrc, separate boundary).

taskfile/ast/task.go

@@ -156,7 +156,7 @@ func (t *Task) UnmarshalYAML(node *yaml.Node) error {
 			Internal      bool
 			Method        string
 			Prefix        string
-			IgnoreError   bool `yaml:"ignore_error"`
+			IgnoreError   bool  `yaml:"ignore_error"`
 			Gitignore     *bool `yaml:"gitignore,omitempty"`
 			Run           string
 			Platforms     []*Platform