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

packages/server/vitest.config.ts

@@ -21,6 +21,6 @@ export default defineConfig({
         maxForks: 1,
       },
     },
-    exclude: ["**/node_modules/**", "**/dist/**"],
+    exclude: ["**/node_modules/**", "**/dist/**", "**/.claude/**"],
   },
 });