fix: make the six registration methods atomic under a throwing change-listener (P1-1)

llm.registerAdapter, agents.register, sessions.create, systemPrompt.section,
systemPrompt.tools, and tools.register each mutated state, emitted a change
event, then returned the disposer. In Cordis a synchronous throw before the
effect returns its disposer leaves nothing for the fiber to collect, so a
throwing change-listener leaked the registry entry permanently — HMR/dispose
could not clean it, and the duplicate-name/already-exists check stayed wedged
until restart.

Convert each to the generator-effect pattern already proven in
AgentLoop.create: mutate state, `yield` the disposer that undoes it (collected
before the next step runs, so it is torn down if a later step throws), THEN
emit the change event. The existing duplicate-name throws are unchanged — they
fire before any mutation, so they correctly leak nothing. No public API change:
generator effects are still synchronous SyncEffects and register() keeps
returning its fire-and-forget disposer wrapper.

Tests: a listener-throw rollback test for all six methods — register with a
change-listener that throws, assert the call throws AND the registry is clean
(entry absent; a subsequent listener-free register of the same name succeeds
and contributes exactly once). For systemPrompt (no duplicate-name check) the
two tests assert assembly is clean. Verified each fails against the pre-fix
emit-before-return-disposer form.

packages/agent/src/index.ts

@@ -35,17 +35,22 @@ export class AgentRegistry extends Service {
    * when the calling fiber is disposed. Returns the disposer.
    */
   register(agent: Agent): () => void {
-    const dispose = this.ctx.effect(() => {
+    const dispose = this.ctx.effect(function* (this: AgentRegistry) {
       if (this.store.has(agent.id)) {
         throw new Error(`agent "${agent.id}" is already registered`)
       }
       this.store.set(agent.id, agent)
-      this.ctx.emit('agent/created', agent)
-      return () => {
+      // Yield the rollback BEFORE emitting `agent/created`: a generator effect
+      // collects each yielded disposer before the next step runs, so a
+      // throwing `agent/created` listener rolls the entry back instead of
+      // leaking it (a leak would wedge the duplicate-id check until restart).
+      // The duplicate throw above fires before any mutation — it leaks nothing.
+      yield () => {
         this.store.delete(agent.id)
         this.ctx.emit('agent/disposed', agent)
       }
-    }, 'agents.register()')
+      this.ctx.emit('agent/created', agent)
+    }.bind(this), 'agents.register()')
     // ctx.effect's disposer returns Promise<void>; our disposer API is
     // synchronous fire-and-forget — discard the (always-resolved) promise.
     return () => void dispose()