Update README to reflect actual project functionality

[Copilot]

The README described NANA-OS as an "Agent Teams management tool" with multi-team orchestration, which no longer matched the actual implementation. It also contained extensive technical details (architecture diagrams, deployment commands, env var tables) that added noise without value.

Changes

• Rewritten project description: Now accurately describes NANA-OS as an event-driven Agent orchestration platform that routes external events to agents and drives automated execution
• Updated feature list: Reflects the real feature set — Agent management, multi-source event ingestion (GitHub/GitLab/Gitea webhooks, IMAP email, generic HTTP), subscription-based routing with cron support, isolated container execution, LLM model registry, and MCP server integration
• Removed technical details: Stripped architecture diagrams, startup commands, and configuration tables per the requirement to keep the README to brief functional descriptions only

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