feat(agentstack): add delegation authority checks

[Qyra-One]

Summary

• Adds listDelegations() for filtering delegation grants by owner, agent, scope, and active timestamp
• Adds hasDelegation() to answer whether an agent has active scoped authority for an owner
• Documents the authority-check API in the AgentStack README/PRD

Test plan

• npm --workspace @logicsrc/schemas run build
• npm --workspace @logicsrc/validators run build
• npm --workspace @logicsrc/plugin-core run build
• npm --workspace @logicsrc/agentstack run build
• npm --workspace @logicsrc/agentstack test

Notes

This fills part of AgentStack FR-5: "Authority checks reference active grants."