chore(marketplace): retire Hello Convax

[smackgg]

Summary

• exclude hello-convax from the current Official Marketplace
• let the publication view remove its owned hello-convax-guide Skill as one lifecycle closure
• replace Hello Convax documentation examples with the useful, ready video-timeline Plugin
• advance the Registry sequence floor for the catalog policy change

Why

Hello Convax is only a connection-checking example and does not provide a useful Marketplace capability. Repository policy requires reviewed exclusions instead of deleting published source or immutable historical Releases.

Validation

• bun install --frozen-lockfile --ignore-scripts
• bun run validate
• bun test tooling/marketplace-publication-view.test.js tooling/official-marketplace.test.js tooling/marketplace-preflight.test.js
• bun run marketplace:check
• bun run pack -- --kind plugin --id video-timeline
• bun run marketplace:build
• workspace build, typecheck, tests, and seven companion builds through bun run check
• isolated retry of the one default-timeout test: bun test --timeout 30000 tooling/vendored-host-package-closure.test.js

The generated Registry v2 and Showcase v2 contain neither hello-convax nor hello-convax-guide.