docs: present product capabilities in README

[sam2tom]

Summary

• rewrite the English and Chinese READMEs around product capabilities and user workflows
• add Dashboard, Agents, Sessions, and System screenshots generated from isolated fixture data
• move implementation-heavy detail behind focused architecture, connection, and protocol documentation links

Validation

• isolated screenshot fixture reset and capture: 1 passed
• screenshots visually inspected at 1440×960
• git diff --check: passed

Data and execution boundary

• no real Core/PostgreSQL data was deleted
• no production data or credentials appear in screenshots
• no paid model call was made