Finish honest Studio Events and pack workbench after Ship this #2491

Goal

Finish the remaining Studio IDE playflow from PR #2439 after #2437 shipped Ship this and closed issue #2435. Keep the existing agent-workflow-ui feature flag for Act, its workflow, and results, as Hayden chose for the related Studio work on 2026-10-28. Issue #2440 separately covers pricing copy and the review-card Act CTA.

Acceptance criteria

• An analyzed saved Video Pack with no extracted events shows honest empty Events copy and retains architecture, artifacts, and named tools in the promoted pack workbench; a tool-only pack must not show an empty workbench.
• Keep Retry transcript available on a failed transcript run, and keep Ship this as the pack ZIP action without inventing events or claiming unrunnable stack checks passed.
• Show runnable stack checks as pending for an authenticated viewer and as skipped for an anonymous viewer; non-runnable named tools remain unknown.
• Enable Act on this run only behind agent-workflow-ui and only for a matching completed stored Video Pack. Bind the trigger, in-flight result, review cards, and export to that selected pack; discard late results after a pack switch.
• Run the affected web tests, lint, type check, production build, and hosted checks on the final PR head. Resolve review findings before merging.

Implementing PR: #2439. This issue tracks its remaining delta and does not reopen the already completed #2435 or close #2440. Local fixes passed 1,530 web tests with 28 skipped, lint, type check and build after merging current main; branch push and hosted validation are pending.