Rename the Taskfile/task option from `gitignore` to `use_gitignore`
to avoid ambiguity (could be read as "ignore git" vs "use .gitignore").
Consistent with Biome's `useIgnoreFile` naming convention.
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).
When `gitignore: true` is set at the Taskfile or task level, files
matching .gitignore rules are automatically excluded from sources and
generates glob resolution. This prevents rebuilds triggered by changes
to files that are in .gitignore (build artifacts, generated files, etc.).
Uses go-git to load .gitignore patterns including nested .gitignore
files, .git/info/exclude, and global gitignore configuration.
