Agents can see their actor kind

The credential lookup, GET /api/me, reported actor_kind only when a signed token carried it explicitly. Every API key and every dashboard session read null, including the key an agent receives when it registers itself, even though the same response named the agent.

actor_kind is now always filled in. Any credential bound to an agent reports agent, whatever else its token says. Otherwise a token's own value is used when it states one, then api_key for any other API key, tenant for a person's token or a dashboard session, and service for the X-Tenant-ID header, which is accepted in local development only.

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