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/App.svelte

@@ -3852,11 +3852,15 @@
     }
   }
 
-  async function saveStash(message: string, includeUntracked: boolean) {
-    if (!activeRepoPath || changedFiles.length === 0) return;
-    const stashedFiles = changedFiles.length;
+  async function saveStash(message: string, includeUntracked: boolean, files?: GitFileStatus[]) {
+    const targets = files ?? changedFiles;
+    if (!activeRepoPath || targets.length === 0) return;
+    const paths = files
+      ? [...new Set(files.flatMap((file) => file.old_path ? [file.old_path, file.path] : [file.path]))]
+      : undefined;
+    const stashedFiles = targets.length;
     await runOperation("Stashing changes", async () => {
-      applyStatus(await stashPush(activeRepoPath, message, includeUntracked));
+      applyStatus(await stashPush(activeRepoPath, message, includeUntracked, paths));
       await refreshRepositoryViews(activeRepoPath, {
         branches: false,
         stashes: true,
@@ -3865,10 +3869,16 @@
       trackEvent("stash_saved", {
         include_untracked: includeUntracked ? 1 : 0,
         changed_files: stashedFiles,
+        scoped: files ? 1 : 0,
       });
     });
   }
 
+  async function stashStatusFiles(files: GitFileStatus[], label: string) {
+    const suffix = files.length === 1 ? files[0].path : `${label} (${files.length} files)`;
+    await saveStash(`Gitty: ${suffix}`, true, files);
+  }
+
   async function applyStashEntry(stash: GitStash) {
     if (!activeRepoPath) return;
     await runOperation(`Applying ${stash.selector}`, async () => {
@@ -5367,6 +5377,7 @@
             onUnstage={unstageFile}
             onDiscard={discardFiles}
             onDiscardMany={discardChanges}
+            onStash={stashStatusFiles}
             onPatch={openPreferredFileDiff}
             onStageAll={stageAllFiles}
             onUnstageAll={unstageAllFiles}