docs: landing README — banner hero, badges, synced translations (RU/ES + new ZH)

[AbsoluteMode]

What

Reworks the README into the landing-style page drafted on codex/landing-readme, and brings all four language versions to identical content: English (reference), Русский, Español, and a new 中文 translation.

English master

• Landing skeleton from the Codex draft: hero banner, badges, capability/tool tables, two mermaid diagrams, collapsible details for per-agent install, transports, licensing.
• One hero image instead of logo + banner — the OG card already carries the logo and tagline. Recompressed site/public/og.png (1.7 MB) → assets/og.webp (106 KB, visually identical), committed on this branch so the README doesn't depend on the site branch merging.
• Folded back load-bearing content from main that the draft had dropped: model licensing notes (CC BY-NC trap), the manual SessionStart hook JSON, all three team-mode transports (shared folder / LAN relay / internet relay + Caddy), metadocs directory structure and server-side mechanics, sidechain skipping, vector reuse, Cursor 2.5+ requirement, setup --lang --yes.
• Team mode kept as a full, prominent section (dialogue example + mechanics + transports) per earlier feedback that it was underrepresented.
• Dropped the stars badge; kept license / python / MCP / tests with the brand colors.
• Every CLI command and flag verified against src/session_recall/ (--scope, share/metadocs subcommands, health output format).

Translations

• docs/README.ru.md, README.es-ES.md rewritten to mirror the master 1:1; README.zh-CN.md added.
• All 4 files: 15 sections, 8 details blocks, 42 code fences — code/console/mermaid blocks byte-identical to the master; language switcher + "English is the reference" note in each translation.
• Internal anchors point at translated headings (GitHub preserves Cyrillic/CJK/accents in slugs) — verified rendered on this branch: all anchors resolve, all images load, both mermaid diagrams render, no broken emphasis around CJK punctuation.

Notes

• The Spanish community translation is superseded by the synced version; the original contribution stays in git history.
• session-recall.es-ES credit line and the old RU-only structure are replaced by the unified layout.

🤖 Generated with Claude Code