The built-in AI agent chat over wiki content is fully implemented
(server module, ~40 tools, RAG search, provider settings, external MCP),
so showcase it in both READMEs:
- list AI agent chat among the from-scratch community replacements
- add a "What's different" table row and a dedicated feature section
- move AI chat from the "In progress" roadmap bucket to "Done"
- add it to the Features list
- add docs/screenshots/ai-chat.png and show it in the Screenshots section
Updated in sync in README.md and README.ru.md.
