perf: share upstream fetches within each run

[flexdinesh]

• Share fetch results across eligible worktrees; keep status and sync independent.
• Fetch fresh each invocation; retain all-remotes behavior and conservative fallbacks.
• Skip sync after fetch failure; show stale/skipped outcomes in both modes.
• Validation: mise run test and mise run check passed, including race detection, vet, formatting, module tidiness, and build.