docs: rework README features section with inline video demos

[quiet-node]

Description

Reworks the README so the Features section shows the product instead of describing it, and trims the doc to match how comparable open-source apps structure their READMEs. Net length drops from ~1830 to ~910 words.

Key changes

• Features: a 2x3 grid of inline, playable demo videos (summon, highlight-to-quote, /screen, web search, model library) plus a slash-commands screenshot, replacing the long bullet list. Videos are GitHub-hosted attachments so they play inline on github.com.
• Why Thuki: rewritten around concrete differentiators (highlight-to-quote in one gesture, zero-setup bundled engine, /screen vision context) and drops an overstated competitor claim.
• Trimmed low-precedent sections: Architecture collapsed to a one-paragraph pointer, roadmap reduced to a single line, community ports folded into Contributing.
• Install: slimmed to the one-line installer; manual DMG, Nightly, and build-from-source moved into a new self-contained docs/install.md.

Notes

• The five feature videos are uploaded GitHub attachments, not committed files, so they render as inline players on github.com. The slash-commands panel is a static screenshot at docs/assets/slash-commands-v2.png.