Summary

• Make the desktop metadata git-branch helper return cached or stale branch data immediately instead of waiting for git on the hydration path.
• Refresh expired or missing git branch metadata in the background, while keeping WorkspaceChanges on the synchronous fresh git path.
• Update MetaForTab and workspace branch-cache tests for the async metadata contract.

Context

This addresses one likely contributor to #5909 where tab.hydrate meta can take around 2s during topic activation. The PR does not close the issue because the thread contains multiple performance-lag shapes.

Tests

• cd desktop && go test . -count=1 -run '^(TestMetaForTabIncludesWorkspaceContext|TestWorkspaceGitBranch|TestWorkspaceGitBranchForMeta|TestWorkspaceGitDisablesDaemonSpawns)'
• git diff --check
• cd desktop && go test ./...