Parallelize git ls-remote calls in UpdateChecker

[bguidolim]

Context

UpdateChecker.checkPackUpdates() runs git ls-remote serially for each pack. With N packs, this takes O(N × RTT). The SessionStart hook has a 30-second timeout — with 5+ packs on a slow connection, serial calls could exhaust the budget.

Currently mitigated by the 7-day cache (network checks are rare), but when they do run, parallelism would reduce latency.

Proposed Change

Use DispatchQueue.concurrentPerform(iterations:execute:) or Swift concurrency (withTaskGroup) to run git ls-remote calls in parallel. Each call is independent — no shared mutable state.

Considerations

• Swift 6 concurrency safety: PackEntry is Sendable (all let fields), ShellRunner needs verification
• Per-process timeout on individual git ls-remote invocations (currently none)
• Thread-safe result collection