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/request-reconstruction.spec.ts

@@ -118,7 +118,7 @@ describe('request stability across the loop', () => {
       preStep()
       const session = agent.session
       const nodes = session.surface.nodes
-      session.append('context/message', {
+      session.append('user/message', {
         content: [{ type: 'text', text: '[summary of turn 1]' }],
         source: { kind: 'plugin', plugin: 'test-compact' },
       }, {
@@ -180,7 +180,7 @@ describe('request stability across the loop', () => {
     const first = adapter.requests[0]!
     // The inject landed in the log after the boundary: not in THIS request…
     expect(first.messages.some(m => m.content.some(b => b.type === 'text' && b.text.includes('[late context]')))).toBe(false)
-    expect(agent.session.events.some(e => e.type === 'context/message')).toBe(true)
+    expect(agent.session.events.some(e => e.type === 'user/message' && e.data.source.kind === 'plugin')).toBe(true)
 
     send(agent, 'second')
     await waitForIdle(ctx, agent)