feat: confirm dialog before the collection Contribute (share) button launches the agent

## Summary The Contribute (share) button on an Installed collection card currently launches the contribute chat **immediately** on click — the agent starts exporting + opening a GitHub PR with no confirmation. A stray click shouldn't kick off a share. Add a confirm dialog first: "Share the \"{title}\" collection? This runs a skill that exports it and opens a GitHub PR to the registry." Only on confirm does `startChat` fire. ## Scope - `CollectionsIndexView.vue`: `startContributeChat` awaits `cui.confirm(...)` before `cui.startChat(...)`. - i18n: `collectionsView.contributeConfirm` (message) across all 8 locales; reuse `collectionsView.contribute` as the confirm CTA. - e2e: confirm-path (one launch), cancel-path (no launch), keyboard-path. - Version bump `@mulmoclaude/collection-plugin`. Follow-up to #1828 (Contribute button), now merged.