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/tests/invariant.spec.ts

@@ -56,3 +56,41 @@ describe('agent status invariants', () => {
     expect(() => { ctx.emit(scopeTarget(b, b), 'agent/status', b, 'running') }).not.toThrow()
   })
 })
+
+describe('agent inbox invariants', () => {
+  const info = (steering: boolean) => ({ content: [], source: { kind: 'user' as const }, contexts: [], steering, wakeup: true })
+
+  it('accepts a dequeue and a discard covered by prior enqueues', async () => {
+    const ctx = await setup()
+    const agent = mockAgent('i1')
+    const at = scopeTarget(agent, agent)
+    expect(() => {
+      ctx.emit(at, 'agent/inbox/enqueue', agent, info(false))
+      ctx.emit(at, 'agent/inbox/enqueue', agent, info(true))
+      ctx.emit(at, 'agent/inbox/dequeue', agent, info(false))
+      ctx.emit(at, 'agent/inbox/discard', agent, [info(true)])
+    }).not.toThrow()
+  })
+
+  it('rejects a dequeue with no outstanding item', async () => {
+    const ctx = await setup()
+    const agent = mockAgent('i2')
+    expect(() => { ctx.emit(scopeTarget(agent, agent), 'agent/inbox/dequeue', agent, info(false)) })
+      .toThrow(/without a matching prior enqueue/)
+  })
+
+  it('rejects a discard larger than the outstanding count', async () => {
+    const ctx = await setup()
+    const agent = mockAgent('i3')
+    const at = scopeTarget(agent, agent)
+    ctx.emit(at, 'agent/inbox/enqueue', agent, info(false))
+    expect(() => { ctx.emit(at, 'agent/inbox/discard', agent, [info(false), info(true)]) })
+      .toThrow(/dropped 2 items but only 1 were outstanding/)
+  })
+
+  it('accepts an empty discard against a fresh agent', async () => {
+    const ctx = await setup()
+    const agent = mockAgent('i4')
+    expect(() => { ctx.emit(scopeTarget(agent, agent), 'agent/inbox/discard', agent, []) }).not.toThrow()
+  })
+})