feat: release ContentCloud v0.5.0

[wutongci]

Summary

• publish the curated ContentCloud Marketplace, signed video-production Scene Plugin, canonical Skills, and bundled MCP
• add the signed Creative Environment control plane, execution bundles, preparation lifecycle, and automation isolation
• align CLI, Web, npm, Plugin, Registry, bootstrap, docs, and deployment configuration on v0.5.0
• make environment preparation fail closed and CreativeExecutionBundle immutable in PostgreSQL

Verification

• make check
• go test -race ./...
• pnpm --dir web test --run (19 tests)
• pnpm evaluate:plugin (8 scenarios)
• native PostgreSQL 18 integration suite with all migrations, RLS, and immutability checks
• staged secret scan and git diff --check

Release gates

• Plugin Registry is Ed25519-signed and source validation passes with zero warnings
• check:plugin --tagged runs after the immutable v0.5.0 tag exists
• npm and production deployment remain separate confirmed steps