Add Agent Registry adapter scaffold

[mdheller]

Summary

Adds a dependency-free Agent Registry adapter scaffold for AgentTerm.

This PR implements the first tested adapter boundary for issue #4 without pretending that a live registry service is wired yet.

It adds:

• AgentRegistration and ToolGrant data models.
• AgentRegistryBackend protocol.
• InMemoryAgentRegistryBackend for tests and local development.
• AgentRegistryAdapter with identity resolution, tool-grant resolution, revocation checks, and fail-closed denial metadata.
• Tests for registered agent resolution, unknown-agent denial, revoked-agent denial, active tool grants, missing tool grants, and result-to-event conversion.

Boundary decisions

• AgentTerm remains the operator surface and event log.
• Agent Registry remains the authority for agent specs, identities, sessions, tool grants, revocation, and runtime authority.
• Unknown, revoked, missing, or ungranted requests fail closed.
• This does not add live network integration yet; it creates the contract and test harness for that next step.

Validation

Expected CI:

ruff check .
pytest

Part of #4.