governance: add tenant guardrails, tool authorization, IM permissions, and budgets

[XnLemon]

Scope

Implement tenant governance and safety controls.

Acceptance

• Tenant-scoped Plugin/Guardrail/Callback chains.
• Tool allowlists, authorization, secret injection, dangerous-operation confirmation.
• IM permissions and sensitive-data masking.
• Atomic token/spend budgets with audit facts.
• Cross-tenant bypass attempts fail closed.