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/tools/src/index.ts

@@ -140,17 +140,22 @@ export class ToolRegistry extends Service {
    * with the calling fiber. Emits `tools/change` on register/unregister.
    */
   register(definition: ToolDefinition): () => void {
-    const dispose = this.ctx.effect(() => {
+    const dispose = this.ctx.effect(function* (this: ToolRegistry) {
       if (this.store.has(definition.name)) {
         throw new Error(`tool "${definition.name}" is already registered`)
       }
       this.store.set(definition.name, definition)
-      this.ctx.emit('tools/change')
-      return () => {
+      // Yield the rollback BEFORE emitting `tools/change`: a generator effect
+      // collects each yielded disposer before the next step runs, so a throwing
+      // `tools/change` listener removes the tool instead of leaking it (a leak
+      // would wedge the duplicate-name check until restart). The duplicate
+      // throw above fires before any mutation — it leaks nothing.
+      yield () => {
         this.store.delete(definition.name)
         this.ctx.emit('tools/change')
       }
-    }, 'tools.register()')
+      this.ctx.emit('tools/change')
+    }.bind(this), 'tools.register()')
     // ctx.effect's disposer returns Promise<void>; our disposer API is
     // synchronous fire-and-forget — discard the (always-resolved) promise.
     return () => void dispose()