Add registration and revocation

What does this MR do and why?

Implements the agent identity registration API for GAIG Stage 1

An agent identity is a runtime governance record representing a specific instance of an external AI agent: a particular agent type (claude-code, opencode) running on a particular machine, authenticated as a particular user, against a particular project. This is distinct from an agent type definition in the AI Catalog, the catalog is a product registry, identities are audit anchors that make every session traceable and revocable at the individual machine level.

glab calls this endpoint silently on the first invocation inside a new agent session. The returned identity ID is cached locally and supplied on all subsequent session creation requests via agent_identity_id.

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Related to #603405