AgentCore (the Core interface impl) belongs in internal/agent alongside
the loop, session, and state it orchestrates — not in a separate one-file
package.
To break the resulting import cycle (agent → pkg/core → agent), define
Event and EventHandler natively in pkg/core instead of aliasing from
internal/agent. The agent loop now imports pkg/core for these types.
Usage token tracking is internal to the loop and no longer emitted as
an event (no consumer used it).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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