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/tests/tools.spec.ts

@@ -147,6 +147,27 @@ describe('ToolRegistry', () => {
     dispose()
     expect(ctx.tools.schemas().map(t => t.name)).toEqual(['echo'])
   })
+
+  it('rolls back the tool entry when a tools/change listener throws (P1-1)', async () => {
+    const ctx = await setup()
+
+    let threw = false
+    ctx.on('tools/change', () => {
+      if (!threw) { threw = true; throw new Error('boom change listener') }
+    })
+
+    // The throwing emit must roll the entry back, not leak it.
+    expect(() => ctx.tools.register(echoTool)).toThrow('boom change listener')
+    expect(ctx.tools.get('echo')).toBeUndefined() // rolled back, not leaked
+    expect(ctx.tools.schemas()).toHaveLength(0)
+
+    // A subsequent listener-free register of the SAME name succeeds and is
+    // exposed exactly once (the duplicate-name check is not wedged).
+    const dispose = ctx.tools.register(echoTool)
+    expect(ctx.tools.schemas().map(t => t.name)).toEqual(['echo'])
+    dispose()
+    expect(ctx.tools.get('echo')).toBeUndefined()
+  })
 })
 
 describe('defineTool / schema DSL', () => {