docs: add Cursor Cloud specific instructions to AGENTS.md

[victorDigital]

Adds a ## Cursor Cloud specific instructions section to AGENTS.md with environment-specific context for future Cloud Agents:

• Documents pre-installed system dependencies (ffmpeg, Node 22, pnpm 10.30.3)
• Notes the cold-encode delay (~60-80s) on first dev server page load
• Quick-reference command table
• Documents known gotchas (engines.node aspirational, .js extension convention, husky hook)

Verification

The development environment was fully set up and validated:

• pnpm install — installs all dependencies and builds library dist/
• pnpm check — type-checks pass (0 errors, 0 warnings)
• pnpm --filter enhanced-video-sveltekit build — library compiles cleanly
• pnpm dev — Vite dev server on :5173, videos encode and serve correctly
• pnpm build — production build succeeds

Demo page fully loaded with video grid
demo_app_video_showcase.mp4

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