docs(README): add hero video — theme-aware cover linking to the 90-second overview (#11941)

* docs(README): add hero video — theme-aware cover linking to the 90-second overview

Insert a theme-aware cover (dark/light via #gh-dark-mode-only /
#gh-light-mode-only) immediately after the chips line, linking to the
official 90-second overview on YouTube (https://youtu.be/CX_FlOoOtr0).
Covers copied from the private steedos-labs/video-studio repo at
projects/2026-08-objectstack-hero-overview/covers/cover-en-{dark,light}.png,
renamed to hero-cover-{dark,light}.png. Docs-only; no changeset.

* fix(README): single dark cover + clickable-video caption, drop light cover

Maintainer viewed the rendered README on github.com and saw both covers
stacked — GitHub's own rendered HTML (Accept: application/vnd.github.html)
emits both <img> tags with the #gh-dark-mode-only / #gh-light-mode-only
fragments intact and applies no theme CSS, so both render unconditionally.

- Keep the dark cover only; drop the #gh-fragment from its src (a single
  image left fragmented would hide it from any renderer that DOES honour
  the mechanism).
- Add a caption link below the image ("▶ Watch: ObjectStack in 90 Seconds")
  so mobile web, desktop, npm's README render, and plain-text readers all
  get a signal that the image is a clickable video, not a decorative
  banner — none of those surfaces render a play button over a plain <img>.
- git rm docs/screenshots/hero-cover-light.png: no longer referenced
  anywhere in the repo (verified via git grep before removal).

README.md diff vs main is still a pure insertion — no pre-existing line
touched.

---------

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>

Author: os-zhuang

README.md

@@ -21,6 +21,14 @@
 
 `Fits in an agent's context` · `Typed, validated, governed` · `Self-host anywhere` · Apache-2.0
 
+<p align="center">
+  <a href="https://youtu.be/CX_FlOoOtr0">
+    <img src="docs/screenshots/hero-cover-dark.png" width="900" alt="ObjectStack in 90 Seconds — watch the overview on YouTube">
+  </a>
+  <br>
+  <a href="https://youtu.be/CX_FlOoOtr0"><b>▶&nbsp; Watch: ObjectStack in 90 Seconds</b></a>
+</p>
+
 **Everything in this repo is the open stack** — protocol, microkernel, SDK,
 CLI, and the production runtime, Apache-2.0 with no open-core asterisks. The
 workflow here is **build & ask with Claude Code** (or any coding agent): the