refactor: eliminate getCheckoutStatusLite, use WorkspaceGitService

Route all git checkout queries through WorkspaceGitService instead of
shelling out to git on every call. This makes fetch_agents_request
instant when warm (cached in-memory snapshots) and deduplicates
concurrent cold-start refreshes via ensureWorkspaceTarget.

- buildProjectPlacementForCwd now uses workspaceGitService.getSnapshot()
- worktree-session call sites use WorkspaceGitService
- Deleted getCheckoutStatusLite and its 4 return types from checkout-git
- Fixed cold-start thundering herd: getSnapshot deduplicates via
  ensureWorkspaceTarget instead of spawning parallel git processes
- Extracted checkout-diff logic into WorkspaceGitService

vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    exclude: ["**/.claude/**"],
+  },
+});