fix(agents-chat-starter): spawn entities via /_electric/entities

[adityavkk]

Spawning an agent in the chat starter PUTs to ${AGENTS_URL}/${type}/${id}. Since the routing refactor in #4307, that path falls through to the durable-streams proxy on the agents-server. The proxy creates a raw stream and returns 201, so the starter's putRes.ok check passes, but no entity is created and the agent never wakes. The visible result is that rooms are created and no philosopher ever replies, with no error anywhere. Full analysis in #4687.

This changes the spawn fetch to the entity API at /_electric/entities/${type}/${id}, the same path ctx.spawn (runtime-server-client.ts), the electric-ax CLI (entity-api.ts), and the conformance DSL use. It also fixes the same snippet in the starter's AGENTS.md.

The entityUrl stored on the room and shown in the UI stays /${type}/${id}. Only the fetch URL changes.

Tested against @electric-ax/agents-server 0.6.3 with embedded streams. Before the change, the spawn PUT returns 201 and /_electric/entities stays empty. After it, the entity is created, the dispatch webhook is registered, and the first wake runs the handler.

No changeset, since the change only touches the example. #4217 is the precedent.

Fixes #4687