docs(readme): tighten intro, add promo video, table-based quick start

[Yanami-2K]

README-only change, in both README.md and README.zh-CN.md, plus the promo video assets. No code or other docs are touched.

What changed

• Intro: the opening now says what LoopForge is. It installs into the coding agent you already use (CodeBuddy, Codex, Cursor, Claude Code or Pi) and runs a role-based workflow inside it: clarify and confirm the requirement, design, implement, independent Code Review by a different role, test. Stages run one after another; you don't run several agents side by side.
• Promo video near the top: the 13 s English promo.
  • GitHub strips <video> for files in the repo (I checked against the markdown render API), and the blob view can't play a committed MP4. So the README embeds an animated WebP preview (docs/assets/loopforge-promo-preview.webp, 960×540, 2.1 MB), which plays inline.
  • Clicking it opens the full MP4 with sound (docs/assets/loopforge-promo.mp4, 1280×720 H.264/AAC, 2.8 MB).
• Quick start: the four per-agent Classic blocks became one table (install command plus start command), with one worked example (Claude Code). The status and doctor commands are kept.
• New "Supported agents" matrix (Classic vs. Portable). Pi is Portable-only.
• Portable Edition uses the same table format, including the Pi pi install npm:pi-subagents step.
• Update/uninstall: the 4+5 repeated per-agent blocks are now the generic loopforge [skills] update|status|uninstall <agent> form, with the agent names listed.
• Kept as-is: artifacts, resume commands, the observability dashboard section and its image, "when it's useful", install state / --force / loopforge plan, pipx install ., tcmcp, security, community QR, contributing, license.
• Wording: "multi-agent" became "role-based" to match how the workflow actually runs inside one host.

scripts/validate.sh and scripts/scan-secrets.sh pass locally.

Rendered README (top)

English	简体中文

In a browser I checked that the preview loads and animates on the rendered page for both languages.