refactor(agent): remove message metadata channel

packages/core/tools/tests/tools.spec.ts

@@ -878,7 +878,7 @@ describe('ToolRegistry', () => {
       description: 'composite',
       parameters: {},
       async execute(_args, exec) {
-        exec.deferContext({ content: [{ type: 'text', text: 'nested-1' }], source: { kind: 'plugin', plugin: 'nested-1' }, meta: { n: 1 } })
+        exec.deferContext({ content: [{ type: 'text', text: 'nested-1' }], source: { kind: 'plugin', plugin: 'nested-1' } })
         exec.deferContext({ content: [{ type: 'text', text: 'nested-2' }], source: { kind: 'plugin', plugin: 'nested-2' } })
         return [{ type: 'text', text: 'done' }]
       },
@@ -912,7 +912,6 @@ describe('ToolRegistry', () => {
       { kind: 'plugin', plugin: 'wrapper' },
       { kind: 'plugin', plugin: 'post' },
     ])
-    expect(result.additionalContexts?.[0]?.meta).toEqual({ n: 1 })
   })
 
   it('keeps deferred contexts when a composite tool throws, but drops them when the outer call is blocked', async () => {