Migrate agents.id references to agents.uuid

Change DB-internal references to the agents table from the string id to the uuid column.

- FK columns: kernels.agent, sessions.agent, agent_resources.agent_id, agent_installed_images.agent_id (composite PKs included).
- Unconstrained arrays: sessions.agent_ids, sessions.designated_agent_ids.
- At read points where a stored reference feeds a runtime key (RPC pool, Valkey, events), convert uuid to the string agent id; the runtime key scheme itself is unchanged.

JIRA Issue: BA-7214

Start by locating the database definitions and read paths for kernels.agent, sessions.agent, agent_resources.agent_id, agent_installed_images.agent_id, sessions.agent_ids, and sessions.designated_agent_ids. Trace where these references become RPC pool, Valkey, or event runtime keys; done means internal references use agents.uuid while those runtime keys still use the string agent id.

Written by the indexing model from the issue text.