🤖 fix: VS Code dock activity pump follow-ups (restart after natural end, narrower task slice) #5658

Follow-up from the final readiness check of #5657 (#5109, webview sub-agent tasks strip and the selected + descendants activity pump). None of these blocks #5657.

1. Activity pump restart after it ends by itself: if the activity subscription ends or fails on its own, the pump stops but its controller is not aborted, so the unchanged-set check in updateActivityPump blocks a restart until the selection, a refresh or membership changes. On main, the old pump restarted with the chat subscription. Fix: abort the controller (or clear the key) when the pump exits.
2. Stale comment in vscode/src/extension.ts ("Stops the sibling activity pump too…") near the chat subscription teardown: the activity pump no longer shares that controller. Remove it.
3. Narrow workflowTask in the host's task slice to the fields the strip reads (runId, workflowName), so outputSchema stays out of every list post and per-event projection key.
4. Lifecycle test: assert activityLists === 2 after the restarted pump's post, so "a rename does not restart the pump" is checked directly.
5. Profile the webview strip's derivation from the full workspace list during streaming at ~4.5k workspaces.

Owner: issue coordinator lane (workspace-74). Trigger: after #5657 merges.

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