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/session/src/index.ts

@@ -167,15 +167,21 @@ export class SessionStore extends Service {
     const sessionId = SessionId(id ?? `session-${++this.counter}`)
     if (this.store.has(sessionId)) throw new Error(`session "${sessionId}" already exists`)
     const session = new Session(sessionId, seed)
-    this.ctx.effect(() => {
+    this.ctx.effect(function* (this: SessionStore) {
       session.onAppend = (event) => { this.ctx.emit('session/event', session, event) }
       this.store.set(sessionId, session)
-      this.ctx.emit('session/created', session)
-      return () => {
+      // Yield the rollback BEFORE emitting `session/created`: a generator
+      // effect collects each yielded disposer before the next step runs, so a
+      // throwing `session/created` listener detaches onAppend and removes the
+      // store entry instead of leaking them (a leak would wedge the
+      // already-exists check until restart). The duplicate throw above fires
+      // before any mutation — it leaks nothing.
+      yield () => {
         session.onAppend = undefined
         this.store.delete(sessionId)
       }
-    }, 'sessions.create()')
+      this.ctx.emit('session/created', session)
+    }.bind(this), 'sessions.create()')
     return session
   }