feat: add mobile agent pairing invite

[1lystore]

Summary

• add dcp-agent mobile pair to generate signed DCP Mobile pairing invites
• print terminal QR/pairing URL for mobile scan or paste
• store pending mobile agent-side key material with restricted file permissions
• add tests for invite shape, signature verification, and MVP scope validation

Validation

• pnpm --filter @dcprotocol/agent test
• pnpm --filter @dcprotocol/agent typecheck
• pnpm --filter @dcprotocol/agent build

Notes

• Draft only. This does not merge mobile work into main.
• Next slice should complete mobile approval/relay exchange and promotion from pending mobile config to runnable agent config.