feat(git): add ignore and untrack paths commands

The changes introduce server-side commands to manage gitignore
 rules and to untrack paths without deleting local files.
 A new GitIgnoreKind enum and helper functions normalize targets
 and build proper ignore patterns, and UI code was wired to use
 these commands.

- add_to_gitignore command and related helpers
- untrack_paths command to remove paths from the index
- UI wiring to expose ignore and untrack actions in explorer

CHANGELOG.md

@@ -18,7 +18,11 @@ The project uses calendar-style versions in the form `YYYY.M.PATCH`.
   existing file actions.
 - Files and folders in the Changes panel now have context-menu actions for
   staging or unstaging their scope and for creating a stash containing only
-  the selected file or folder.
+  the selected file or folder. New and untracked items can also be added to
+  the repository `.gitignore` from Changes or the File Explorer as an exact
+  file, a complete folder, or an extension-wide pattern. Folder rules are only
+  offered for folder selections. Tracked files and folders can be removed from
+  the Git index without deleting their working-tree contents.
 - Gitty can open a repository directly at startup through the `--repo PATH` or
   `--repo=PATH` command-line argument.