docs: simplify README demo media

[mijun (houlianpi)]

Summary

Follow up on merged PR #74 by simplifying the README demo section. Remove the low-signal GIF preview and make the embedded video the primary demonstration.

Related issue

No issue required. This is a documentation presentation change following review of the merged README structure.

Design document

No design document required. This changes release-media presentation only and preserves product behavior and public contracts.

SPEC updates

No SPEC files changed. Reviewed SPEC.md and fsq_agent/cli/SPEC.md; the change does not affect runtime behavior, CLI contracts, configuration, or module boundaries.

Changes

• Remove the GIF preview from the English and Chinese README demo sections.
• Add a short sentence explaining that the video covers the goal, live UI execution, evidence, reviewable Case, and deterministic replay.
• Keep the GitHub attachment as the inline video and add an explicit YouTube link.
• Update the distribution contract so it continues to protect both video URLs without requiring the removed GIF.

Verification

• git diff --check — passed.
• uv run --no-sync python -m pytest tests/test_distribution_contract.py -q — 12 passed.

Evidence

N/A. This PR removes an existing preview and uses the already approved embedded video asset.

Release or documentation impact

The README demo section now has one primary video instead of a dense GIF plus video. English and Chinese pages remain aligned.

Checklist

Spec-driven development

• I reviewed root SPEC.md and every relevant module SPEC.md.
• Any required design and SPEC confirmation gates occurred before implementation.
• The implementation, tests, and documentation agree with the current confirmed SPEC files.
• I used the current confirmed SPEC files, not the design document, as the implementation source of truth.

Quality and safety

• I ran focused tests and required formatting or lint checks, or explained why a check was not applicable.
• I updated user-facing documentation when behavior or setup changed, or it is not applicable.
• I did not commit credentials, tokens, account data, personal data, local .env files, generated logs, reports, screenshots, or workspace output.
• I reviewed any included screenshots, videos, logs, reports, and Run artifacts for secrets, private paths, device identifiers, and account data.
• I did not add large binary media to Git history unless the repository already owns that exact artifact and the size is justified.