docs: clarify README and remove internal brand docs

[wengzige]

What changed

• remove the public brand-guideline page, machine-readable brand spec, and standalone brand SVG
• keep only the public logo bitmap required by the README, moved into the normal README image directory
• make the online workspace the primary README entry with the first CTA, first full-width product image, and first quick-start flow
• reorganize the remaining content around local-first alternatives, entry selection, benefits, quick starts, advanced capabilities, privacy, and development
• reduce the README from 456 to 337 lines by merging repeated sections and collapsing setup details

Public-content audit

Reviewed every file introduced since 156aea8, the current gh-pages tree, and all uploaded v0.2.2 release assets. No private MCP source, internal planning notes, machine-local paths, tokens, or keys were found outside the removed docs/brand documentation. The Pages branch contains only the built site assets; the MCPB contains only its manifest and two platform binaries.

The removed files remain reachable in existing Git history and the v0.2.2 tag until a separately approved history/tag rewrite is performed.

Checks

• README structural validation (headings, local targets, code fences, HTML tags)
• all existing external links and images return HTTP 200
• npm run typecheck
• npm test -- --run (277 tests passed)
• git diff --cached --check