Fix empty worktree list on first launch

[sapsaldog]

Problem

On first launch, the main window shows "No Worktrees" (0 count) for the selected repository. Switching to another repository loads the list correctly.

Root cause

A cold-start ordering bug between three pieces that are individually correct:

1. At launch, AppDelegate.observeSelectedRepository() syncs worktreeViewModel.repository to the restored selection — intentionally without loading worktrees (the status menu loads lazily; pinned by selectedRepoDoesNotEagerLoadWorktrees).
2. When the main window later opens, ContentView's .task only loaded the list when worktreeViewModel.repository == nil — but step 1 already set it, so the initial load was skipped.
3. .onChange(of: selectedRepository) never fired either, because the selection happened before the view appeared.

Net effect: nothing ever loads the list until the user changes repositories. repository != nil does not imply the list has been loaded — the .task guard assumed it does.

Fix

• New WorktreeListViewModel.windowAppeared(selectedRepository:): always syncs the current selection and reloads when the window content appears.
• ContentView's .task delegates to it instead of the nil-guarded load.
• The AppDelegate sync contract is unchanged.

Testing

• 3 new regression unit tests, including the key case: repository pre-synced (cold-start state) → windowAppeared must still load.
• Red→green was demonstrated against the real bug with a temporary window-hosting integration test: with the old ContentView wiring it times out (list stays empty for 5s after the window opens); with the fix it loads in ~0.2s. The integration test is not kept: AppDelegateColdStartTests has a pre-existing flaky AppKit _NSWindowTransformAnimation over-release crash (crash reports pre-date this change) that any added window-lifecycle test destabilizes — suite hardening is tracked separately.
• swiftlint lint clean, full suite (446 tests) green 3× consecutively, scripts/coverage.sh gate passes.
• Manually verified: worktrees now load on first launch.

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