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/llm/tests/service.spec.ts

@@ -146,4 +146,26 @@ describe('LlmService', () => {
       expect((error as LlmError).code).toBe('DUPLICATE_ADAPTER')
     }
   })
+
+  it('rolls back the adapter entry when an adapter-change listener throws (P1-1)', async () => {
+    const ctx = new Context()
+    await ctx.plugin(LlmService)
+
+    // A change listener that throws on the FIRST emit only.
+    let threw = false
+    ctx.on('llm/adapter-change', () => {
+      if (!threw) { threw = true; throw new Error('boom change listener') }
+    })
+
+    // The throwing emit must roll the mutation back, not leak it.
+    expect(() => ctx.llm.registerAdapter(['m1'], new ScriptedAdapter(SCRIPT))).toThrow('boom change listener')
+    expect(ctx.llm.models()).toEqual([]) // entry rolled back, not leaked
+
+    // A subsequent listener-free register of the SAME model succeeds and
+    // contributes exactly once (the duplicate check is not wedged).
+    const dispose = ctx.llm.registerAdapter(['m1'], new ScriptedAdapter(SCRIPT))
+    expect(ctx.llm.models()).toEqual(['m1'])
+    dispose()
+    expect(ctx.llm.models()).toEqual([])
+  })
 })