docs: document hosted reliability boundaries

[dajiaohuang]

Summary

• turn the repository README into the complete Local/Hosted architecture and operating entry point
• document MCP 2026-07-28 request-scoped discovery, per-request authorization, explicit legacy rollback, current-system-only connection, and the Host's maximum 16-tool projection
• align the bilingual developer and start pages with durable room turns, trusted identity delegation, standard MCP media preservation, receipt-driven projections, negotiated MCP Tasks, bounded workspaces, observability, and release-lock v3
• keep archived split repositories out of installation, release, and rollback paths

Validation

• npm ci
• npm run check
• npm run check:external (17 unique URLs, 0 warnings)
• git diff --check
• secret-pattern scan of all changed files

The first external-link invocation was intentionally retried after the concurrent build completed because it started before dist/ existed; the post-build run passed.