feat(git): add stash push with optional paths and per-file scope

The stash push feature now supports limiting the stash to
selected files via an optional paths parameter.
The UI passes file paths to stash_push and adds a
per-file context menu for scoped stash operations.

- Extend stash API to accept optional paths for scoped stashes
- Implement per-file stash actions via a status panel context menu
- Update tests and docs to reflect scoped stash behavior

docs/api-contract.md

@@ -138,6 +138,7 @@ The command list below includes the repository-management and synchronization AP
 - `stage_files(path: string, files: string[]): Promise<GitStatus>`
 - `unstage_files(path: string, files: string[]): Promise<GitStatus>`
 - `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>`
 - `fetch(path: string, prune?: boolean, remote?: string): Promise<GitStatus>`
 - `pull(path: string, strategy?: "merge" | "rebase" | "ff-only", remote?: string, branch?: string): Promise<GitStatus>`