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

docs/api-contract.md

@@ -137,6 +137,8 @@ The command list below includes the repository-management and synchronization AP
 - `repair_worktree(path: string, worktreePath: string): Promise<GitWorktree[]>`
 - `stage_files(path: string, files: string[]): Promise<GitStatus>`
 - `unstage_files(path: string, files: string[]): Promise<GitStatus>`
+- `add_to_gitignore(path: string, target: string, kind: "file" | "extension" | "folder"): Promise<GitStatus>`; appends a repository-root `.gitignore` rule and unstages newly-added matching files.
+- `untrack_paths(path: string, targets: string[]): Promise<GitStatus>`; removes files or folders from the Git index while preserving their working-tree contents.
 - `restore_files(path: string, files: string[], staged: boolean): Promise<GitStatus>`
 - `stash_push(path: string, message?: string, includeUntracked?: boolean, paths?: string[]): Promise<GitStatus>`; when `paths` is provided, only matching files are stashed.
 - `commit(path: string, message: string): Promise<GitStatus>`