Fix status pills not rendering on workspaces created after launch

[obvioussean]

Status pills set on a workspace created after app launch are stored in the Tab model but never render in the sidebar — while pre-existing workspaces render immediately. list-status confirms the data; the UI shows nothing.

Why

.onAppear fires synchronously during view insertion, before .onReceive subscriptions are live. Sidebar data set by socket commands (e.g. agent hook set_status) between .onAppear and subscription goes unobserved: the observation publishers use .dropFirst() to skip the current-value replay, making them change-only — so a mutation that lands before the subscriber connects isn't replayed, and the .onAppear snapshot captured the pre-mutation (empty) state. Pre-existing workspaces work because their .onReceive subscriptions were established before any status was set.

What didn't work: removing .dropFirst() globally

The initial approach removed .dropFirst() from sidebarObservationSignal and the $panels chain, turning the publishers from change-only into replay-on-subscribe. This fixed the immediate bug but introduced three problems caught by adversarial review (GPT-5.5 + GPT-5.4, independently):

1. Extension sidebar refresh loop — extensionSidebarDebouncedObservationPublisher is a computed function that creates a new MergeMany per body evaluation. Subscription-time replay → handler increments @State token → body re-eval → new subscription → replay → self-sustaining churn.
2. Immediate publisher fires ~7 un-debounced replays per row on the typing-latency-sensitive path, each calling makeWorkspaceSnapshot().
3. Test breakage — existing tests assume change-only semantics (publishCount == 0 after subscribe + heartbeat mutation).

What works: targeted deferred refresh

Add a one-shot .task to TabItemView that calls refreshWorkspaceSnapshot() — running asynchronously on the next main-actor tick after the view's modifier chain (including .onReceive subscriptions) is fully established. It re-reads tab state at that point, catching any mutations that landed in the .onAppear-to-subscription gap. SidebarWorkspaceSnapshotRefreshPolicy.decision no-ops when nothing has changed, so the cost for pre-existing workspaces is one cheap snapshot comparison per row appearance.

The observation publishers keep their .dropFirst() and change-only contract. No other consumers are affected.

Validation

Couldn't build locally (SPM resolution failures in the worktree) — CI will validate. The change is +11 lines in one file (a .task modifier on TabItemView).

Closes #5659

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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[obvioussean]

Closing in favor of #5662 which fixes this at the publisher level with .prepend(()) — cleaner than my consumer-side .task approach, benefits all subscribers, and includes a regression test.