docs(concepts): add multi-tenant usage guide

[qin-ctx]

Description

Add a bilingual multi-tenant concepts guide that explains account/user/agent boundaries, sharing rules, and the current OpenClaw plugin and Vikingbot integration patterns.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Add docs/zh/concepts/11-multi-tenant.md to explain the multi-tenant model, capability boundaries, and standard usage flow
• Add docs/en/concepts/11-multi-tenant.md with the same structure for English readers
• Link the new concepts doc from the authentication and admin docs in both languages

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Docs-only change; no automated tests were run.

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

This guide explicitly distinguishes the OpenClaw plugin's user-key-based integration from Vikingbot's root-key-managed multi-user pattern so readers do not mix the two practices.

[github-actions]

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