Give provider streams one lifecycle owner (Gates 1-4)

CONTEXT.md

@@ -14,3 +14,22 @@ reasons as completed steps, and mapping thrown errors to known terminal
 provider errors. The agent **runtime** layer starts streams and the **hosted**
 layer finishes them; both consult this module rather than re-deriving the
 interpretation, so provider behavior changes land in one file.
+
+## Stream Lifecycle
+
+The single owner of one provider stream attempt, from the first provider read
+until completion, tool handoff, cancellation, or failure:
+`src/agent/streaming/lifecycle/`. It decodes provider parts through a Provider
+Adapter, reduces them into validated **semantic**, **telemetry**, and
+**diagnostic** frames, owns monotonic provider-wait deadlines and status
+cadence, and settles exactly one typed Stream Outcome per attempt. Telemetry
+observes execution but never extends a semantic deadline. One agent run may
+contain several provider attempts separated by local tool execution.
+
+## Stream Delivery
+
+The separate agent-loop fan-out boundary that will route lifecycle frames to
+live, durable, diagnostic, and usage Adapters (Phase 5, separately designed).
+Through Gate 4, hosted durable and AG-UI production projections still consume
+compatibility UI chunks, and production runs stay on stream protocol
+version 1.