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/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts

@@ -68,7 +68,7 @@ describe('startInProcessRun', () => {
         turn,
         trigger: { kind: 'injection', source: { kind: 'plugin', plugin: 'late-metadata' } },
       })
-      session.append('context/message', {
+      session.append('user/message', {
         content: [{ type: 'text', text: 'late metadata' }],
         source: { kind: 'plugin', plugin: 'late-metadata' },
       }, { surfaceOp: 'append' })

packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts

@@ -187,10 +187,10 @@ describe('dsh-subagent-spawn', () => {
     expect(published).toEqual([])
   })
 
-  it('a cancel from agent/queued maps a no-turn child log to aborted', async () => {
+  it('a cancel from agent/inbox/enqueue maps a no-turn child log to aborted', async () => {
     const { ctx, parent } = await setup([])
     const controller = new AbortController()
-    ctx.on('agent/queued', () => { controller.abort('queued-window') })
+    ctx.on('agent/inbox/enqueue', () => { controller.abort('queued-window') })
     const run = await start(ctx, 'spawn', { prompt: [{ type: 'text', text: 'p' }], parent, signal: controller.signal })
     const result = await run.result
     expect(result).toMatchObject({ stopReason: 'aborted', output: [] })