Add a narrow `agent: { streamFn: StreamFn }` field to the PiAgentSession
interface, sourced structurally from the real AgentSession.agent (Agent,
from the stable @earendil-works/pi-agent-core package). This is the
resolved-auth/headers/retry "call this model" function pi's own
compaction/branch-summarization code uses internally, and will let
sessionNameGenerator.ts drop its dependency on the deprecated pi-ai
/compat provider registry in a follow-up leg.
No adapter object was needed: AgentSession's real .agent field already
structurally satisfies the new narrow shape, so defaultCreateAgentRuntime
needed no changes.
Adds a unit test proving the field is wired end-to-end via the injected
runtime creator. sessionNameGenerator.ts itself is untouched (leg C).
Federico Jaramillo Martinez
6f59a4243e