docs: consolidate adapter docs and discovery loading

[jackwener]

Summary

• consolidate desktop adapter docs under docs/ and remove low-value README duplicates from src/clis
• fix drift in adapter docs for antigravity, chatgpt, codex, cursor, and discord-app
• align filesystem discovery with manifest behavior so only real cli() modules are imported in dev mode

Testing

• pnpm -s tsc --noEmit
• pnpm -s vitest run src/engine.test.ts src/build-manifest.test.ts

[jackwener]

Merged. This consolidates adapter documentation into docs/, removes low-value duplicate README files under src/clis, and aligns dev-time discovery with manifest loading so helper modules are not imported as commands. Thanks for the cleanup push.