Polish agent harness demos and add a unified overview

[AngeLouCN]

The agent replays now make it clear when the LLM or Jev is acting and how much the recorded pair saves. WebShop, FrozenLake, and SQLite use shared robot portraits, one fixed position per controller, and prominent token/time savings. A nine-second overview joins the three three-second clips without changing their frames or timing.

• Add a demo gallery with playback controls below the frame, so paused videos keep the savings visible. It supports seeking, fullscreen, reduced motion, and one replay playing at a time.
• Link the gallery from both READMEs and the English/Chinese homepages, and include its HTML/CSS/JS in the GitHub Pages artifact.
• Keep current main's recorded totals, action-count documentation, golden rules, and localization. No model, experiment, or result changes.

Validation:

• python3 site/tests/content_checks.py — 7 checks passed.
• python3 site/tools/build_content.py and python3 site/tests/check_site.py — clean-checkout build, links, media, and benchmark data passed.
• python3 site/tests/browser_checks.py — homepage playback, all 30 replays, English/Chinese behavior, and responsive layouts passed.
• Gallery playback verified at 1440/900/390 px, including seek/fullscreen, manual pause, reduced motion, no-JS fallback, and offline playback.
• All 183 overview frames and durations match the individual GIFs; all eight MP4/WebM exports decode at 1080×700 with the expected duration.
• GitHub Pages staging includes the gallery and all linked assets; all 274 rebuilt public files match the site validated in Chromium.

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[github-actions]

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Reviewed head 5ff8cfbb7ffb2dce61bb6aab45ed6de1b049c539; base 33cb677bbe300ad853f2d2a061f2e1ea34e7fcba.
CPU CI does not verify production-model quality, CUDA or multi-GPU performance.

[github-actions]

Human-Review. See the maintained PR review summary for evidence.

[github-actions]

Human-Review. See the maintained PR review summary for evidence.