feat(agent): unify send(target × wakeup), coalesce context/message into user/message

Replace send/steer/inject with one Agent.send primitive over the
(target × wakeup) matrix; followup/steer/inject become fixed-preset
alias methods on the now-abstract Agent class. Coalesce context/message
into user/message (injected context is a non-user source). Replace
agent/queued with agent/inbox/enqueue/dequeue/discard, add cancel
keepInbox, and add a FIFO-conservation invariant.

packages/core/agent-loop/tests/inbox.spec.ts

@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest'
 import { Inbox } from '../src/inbox.ts'
 
 function message(text: string) {
-  return { content: [{ type: 'text' as const, text }], source: { kind: 'user' as const }, contexts: [] }
+  return { content: [{ type: 'text' as const, text }], source: { kind: 'user' as const }, contexts: [], wakeup: true }
 }
 
 function resolverPair() {
@@ -25,6 +25,32 @@ describe('Inbox', () => {
     expect(inbox.dequeueQueued()).toBeUndefined()
   })
 
+  it('enqueue(msg, false) queues without waking a parked waiter', async () => {
+    const inbox = new Inbox()
+    let woke = false
+    const waiter = inbox.waitForQueued(new Promise(() => {})).then(() => { woke = true })
+    inbox.enqueue(message('quiet'), false)
+    // The item is queued, but the parked waiter was not resolved by it.
+    expect(inbox.hasQueued).toBe(true)
+    await Promise.resolve()
+    expect(woke).toBe(false)
+    // A later waking enqueue resolves the same waiter.
+    inbox.enqueue(message('loud'))
+    await waiter
+    expect(woke).toBe(true)
+  })
+
+  it('pending() snapshots queued then steering without removing them', () => {
+    const inbox = new Inbox()
+    inbox.enqueue(message('q'))
+    inbox.steer(message('s'))
+    const pending = inbox.pending()
+    expect(pending.map(p => p.steering)).toEqual([false, true])
+    // Snapshot does not drain the FIFOs.
+    expect(inbox.hasQueued).toBe(true)
+    expect(inbox.hasSteering).toBe(true)
+  })
+
   it('pushes and drains steering messages separately from queued', () => {
     const inbox = new Inbox()
     inbox.steer(message('steer'))