docs: sync CLI and README with current MCP/Skills support

[han6569084]

Summary

• document missing CLI commands for skill installation and MCP management
• replace outdated "10 built-in Skills" wording with non-fixed wording
• align the Mimo model name in README with the actual preset (mimo-v2-flash)

Impact

• improves consistency between README.md, docs/cli.md, docs/skills.md, and the current NSH commands
• reduces confusion for users trying to use MCP-related CLI commands
• avoids outdated documentation as the number of built-in skills grows

Testing

• verified documented commands against src/channels/nsh_commands.c
• verified the Mimo preset name against src/channels/cmd_llm.c
• checked current built-in skill files under agent_skills/

[github-actions]

❌ CLA Signature Required

@han6569084 Some contributors need to sign the CLA:

• hanzhijian@zepp.com ❌ Needs to sign CLA

Please:

1. Sign the CLA at: https://www.openvela.com/#/community/cla
2. After signing, comment /check-cla to recheck

📋 View detailed check results: Action Run #26213563065

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💡 Tip: All contributors must sign the CLA before the PR can be merged.

[han6569084]

Superseded by #16 because the contributor email was updated to the requested address. Please review the replacement PR instead.