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

src/lib/git.ts

@@ -332,11 +332,13 @@ export function stashPush(
   path: string,
   message?: string,
   includeUntracked = true,
+  paths?: string[],
 ): Promise<GitStatus> {
   return invoke<GitStatus>("stash_push", {
     path,
     message: message?.trim() ? message.trim() : null,
     includeUntracked,
+    paths: paths?.length ? paths : null,
   });
 }