feat(git): add upstream tracking and local-only branch UI

This release adds upstream tracking for branches and a local-only
indicator in the UI.
The Git integration now exposes upstreams and supports publish flows.
This enables publishing and remote-tracking configuration.

- Introduces upstream tracking and local-only branch UI markers
- Updates toolbar to reflect publish-local state and status indicators
- Bumps version to 2026.8.3 and updates changelog

src/lib/types.ts

@@ -129,6 +129,7 @@ export interface GitBranch {
   name: string;
   current: boolean;
   remote: boolean;
+  upstream: string | null;
 }
 
 export interface GitWorktree {