Fix architecture-review findings in the loop and service packages

High (loop pipeline): agent/step-result now runs before the
assistant/message append so the session log records what tool dispatch
actually uses; abort is honored between tool calls, not just
mid-stream; steering drains at step start, pending steering overrides
a negative turn-continuation decision (/goal pattern), and leftover
steering is re-enqueued as queued messages so it is never stranded;
exceptions from turn-continuation listeners and session/flush are
contained to the turn (error event + agent/error) instead of killing
the driver loop.

Medium: disposal emits agent/status('disposed') and mid-turn disposal
records reason 'disposed'; duplicate LLM adapter registration throws
(all-or-nothing); SessionEvent is a real discriminated union (casts
removed); model-less agents fail with a clear actionable error unless
agent/request supplies a model.

Low: agent/queued and agent/steering carry the resolved MessageSource;
streamBlocks() yields strictly in stream order and flushes delta-only
blocks (matches generate()); BlockAssembler freezes blocks on
block-end and ignores stragglers from malformed streams; turn
numbering is a counter seeded from the log (fork-safe); LoopAgent's
stop disposer is infallible (a throwing status listener cannot skip
registry cleanup); AgentLoop.create uses a generator effect so stop
and unregister are independent disposables; SessionStore wires
onAppend inside its effect.

21 regression tests added (review-fixes.spec.ts), organized by
finding. Docs updated: loop pseudocode (status emissions, ordering,
error containment, steering guarantees) and waterfall composition
caveat in docs/architecture.md; AGENTS.md notes that excessive tests
are welcome.

packages/agent-loop/src/index.ts

@@ -59,14 +59,12 @@ export class AgentLoop extends Service {
   create(id: string, options: AgentOptions = {}): LoopAgent {
     const session = this.ctx.sessions.create(`${id}-session`)
     const agent = new LoopAgent(this.ctx, id, options, session)
-    this.ctx.effect(() => {
-      const stop = agent.start()
-      const unregister = this.ctx.agents.register(agent)
-      return () => {
-        stop()
-        unregister()
-      }
-    }, 'agentLoop.create()')
+    // Generator effect: stop and unregister are independent disposables
+    // (LIFO), so a throwing stop() cannot leak the registry entry.
+    this.ctx.effect(function* (this: AgentLoop) {
+      yield this.ctx.agents.register(agent)
+      yield agent.start()
+    }.bind(this), 'agentLoop.create()')
     return agent
   }
 }