feat(external): register passthrough CLIs under ext namespace

[jackwener]

Summary

• register external CLIs into the unified command registry under the canonical ext/ namespace
• keep curated top-level aliases like opencli gh working by routing them through the same execution path
• surface external CLI metadata in structured opencli list output, shell completion, and docs while preserving passthrough behavior

Testing

• npm run typecheck
• npm test -- --run src/serialization.test.ts src/commanderAdapter.test.ts src/completion.test.ts src/external.test.ts src/execution.test.ts
• node_modules/.bin/tsx src/main.ts ext gh --version
• node_modules/.bin/tsx src/main.ts gh --version
• node_modules/.bin/tsx src/main.ts list -f json | rg ext/gh|ext/docker|ext/vercel