feat(git): add tag management and cherry-pick workflow

This update extends the Git backend and UI to support listing,
creating, deleting, and pushing tags. It also adds cherry-pick
commands with proper in-progress detection and conflict handling, and
prevents other operations while a cherry-pick is active.

- Add GitTag model, tag listing, and tag CRUD/push commands
- Implement cherry-pick start/continue/abort with status tracking
- Update UI to display tags and gate actions during cherry-pick

src/lib/components/HistoryPanel.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { ChevronDown, ChevronRight, EllipsisVertical, GitBranch, GitMerge, RotateCcw, X } from "@lucide/svelte";
+  import { Cherry, ChevronDown, ChevronRight, EllipsisVertical, GitBranch, GitMerge, RotateCcw, X } from "@lucide/svelte";
   import type { FileStatusKind, GitCommit, GitCommitFile } from "../types";
 
   interface GraphSegment {
@@ -40,6 +40,7 @@
     onPreviewCommitFile: (commit: GitCommit, file: GitCommitFile) => void;
     onToggleCommitFiles: (hash: string) => void;
     onCreateBranchFromCommit: (commit: GitCommit) => void;
+    onCherryPickCommit: (commit: GitCommit) => void;
   }
 
   let {
@@ -54,6 +55,7 @@
     onPreviewCommitFile = () => {},
     onToggleCommitFiles = () => {},
     onCreateBranchFromCommit = () => {},
+    onCherryPickCommit = () => {},
   }: Props = $props();
 
   let hiddenGraphBranches = $state<Set<string>>(new Set());
@@ -380,6 +382,13 @@
     await onRestoreCommit(commit);
   }
 
+  async function cherryPickContextCommit() {
+    const commit = contextCommit;
+    if (!commit || isBusy) return;
+    closeCommitContextMenu();
+    await onCherryPickCommit(commit);
+  }
+
   function handleWindowKeydown(event: KeyboardEvent) {
     if (event.key !== "Escape") return;
     closeCommitContextMenu();
@@ -656,6 +665,16 @@
         <RotateCcw size={14} aria-hidden="true" />
         Restore
       </button>
+      <button
+        type="button"
+        role="menuitem"
+        onclick={cherryPickContextCommit}
+        disabled={isBusy}
+        title="Apply this commit's changes on top of the current branch"
+      >
+        <Cherry size={14} aria-hidden="true" />
+        Cherry-pick
+      </button>
     </div>
   {/if}
 </section>