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/agent.spec.ts

@@ -139,7 +139,7 @@ describe('Agent', () => {
     agent.session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
     agent.inject([{ type: 'text', text: 'mid' }], { source: { kind: 'plugin', plugin: 'p' } })
     expect(agent.session.events.filter(e => e.type === 'turn/start')).toHaveLength(1)
-    expect(agent.session.events.at(-1)!.type).toBe('context/message')
+    expect(agent.session.events.at(-1)!.type).toBe('user/message')
 
     // Close the turn; now inject must wrap its own one-shot injection turn.
     agent.session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
@@ -151,6 +151,16 @@ describe('Agent', () => {
     expect(agent.session.events.at(-1)!.type).toBe('turn/end') // turn-enclosed
   })
 
+  it('inject() defaults its source to an empty plugin, never user', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
+    agent.session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    agent.inject([{ type: 'text', text: 'no explicit source' }])
+    const injected = agent.session.events.at(-1)!
+    expect(injected.type === 'user/message' && injected.data.source).toEqual({ kind: 'plugin', plugin: '' })
+  })
+
   it('idle inject() contains a failing flush (logs, does not throw into the caller)', async () => {
     const adapter = new MockAdapter([textResponse('ok')])
     const ctx = await harness(adapter)
@@ -202,7 +212,7 @@ describe('Agent', () => {
 
     expect(() => { agent.inject([{ type: 'text', text: 'notice' }], { source: { kind: 'plugin', plugin: 'p' } }) }).not.toThrow()
     const types = agent.session.events.map(e => e.type)
-    expect(types).toEqual(['turn/start', 'context/message', 'turn/end']) // balanced
+    expect(types).toEqual(['turn/start', 'user/message', 'turn/end']) // balanced
     await new Promise(r => setTimeout(r, 10))
     expect(flushes).toBe(1) // checkpoint fired despite the throwing turn/end listener
   })