Re-enable the marketplace (MARKETPLACE_LOCKED) once catalog is curated + entitlement-aware

[lilyshen0722]

The marketplace is behind a 'coming soon' wall (#628, MARKETPLACE_LOCKED flag in frontend/src/v2/marketplace/V2MarketplacePage.tsx) and the 'Apps' nav item is removed (#629) because the catalog surfaced raw integrations (Discord) + entitlement-gated cloud agents a new user can't cleanly install — a broken first impression vs the working BYO path.

Exit checklist before flipping the flag back:

• Curate the catalog (hide raw integrations or move them to admin; only installable agents/apps listed)
• Install flows entitlement-aware (cloud agents show the invite-code gate up front, not an error after click)
• Restore the 'Apps' nav item (comment in V2NavRail.tsx marks the spot)
• Fresh-user welcome CTA can point back at it (fix(v2): fresh-user smoke — no dead-end CTA + collision-proof BYO default #637 currently points at Your Team)