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/coverage-edges.spec.ts

@@ -47,7 +47,7 @@ describe('inbox acceptance', () => {
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create(SessionId('a1'), { provider: 'mock', model: 'mock' })
     let queued = 0
-    ctx.on('agent/queued', () => { queued += 1 })
+    ctx.on('agent/inbox/enqueue', () => { queued += 1 })
 
     expect(() => {
       agent.send([{ type: 'text', text: 'first', bad: 1n } as never])