docs(readme): lead with the product, not the positioning

[bobleer]

Summary

Restructures both READMEs so a first-time visitor learns what BitFun is, sees it, and can download it without scrolling. Adds the missing social preview image and a script for the demo GIF the README still needs.

No claims were changed — this is reordering, tightening, and two new assets.

Type and Areas

Type: docs

Areas: docs, repo assets

Motivation / Impact

Repo traffic over the last 14 days was 995 unique visitors, and referrers are almost entirely github.com, search engines, and our own site — there is essentially no inbound from developer communities. Before driving traffic anywhere, the landing page has to hold it. Three things were working against that:

1. The download link was at line 87, below the fold and below three benchmark charts. Releases pull 3.3k+ downloads each, so people do find it — but only after scrolling past concept art.
2. The opening was abstract ("a local-first, new-generation foundation for agentic applications"). It reads as positioning, not as a description of a thing you can install.
3. The repo has no social preview image. Every link shared to X, Slack, Discord, or WeChat renders as GitHub's default grey card. png/og-image.png (1280x640) is added here, though applying it still needs a repo admin — Settings → General → Social preview. I only have write access.

Changes:

• Lead with one concrete sentence, the actual app screenshot, and a download link
• Move Install directly under the fold, source build alongside it
• Condense metrics; keep the single-run caveat, promoted to a > [!NOTE] callout
• Move roadmap positioning and the concept diagram below the fold
• Add download/star badges and Discussions links
• Add scripts/record-demo.sh — a 20-30s GIF of a real task is still the highest-impact thing missing from this page, and the script removes the fiddly parts (capture, palette encoding, GitHub's 10 MB limit)

Verification

• node scripts/check-repo-hygiene.mjs → passed (5 content files scanned, 5889 filenames checked)
• bash -n scripts/record-demo.sh → syntax OK
• Every relative link and image target in both READMEs resolved against the working tree — 15/15 exist
• Rendered png/og-image.png at 1280x640, 404 KB (GitHub's limit is 1 MB)
• Docs-only change; no build or runtime checks apply

Reviewer Notes

Two follow-ups this PR cannot cover, both needing repo admin:

• Social preview: upload png/og-image.png under Settings → General → Social preview

• Repo description: the current one reads ...Agent runtimeand a ready-to-use suite of desktop Agent applications.with built-in Code Agent 、 Cowork Agent、Computer Use... — two missing spaces and CJK punctuation in the English one-liner that shows up in search results and every share card. Suggested replacement:

  Open-source desktop AI agent — Code Agent, Cowork, and Computer Use on a local-first Rust runtime. Cross-platform Tauri app for macOS, Windows, and Linux.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.