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.

examples/acp-agent/tests/goal.snapshot.ts

@@ -83,6 +83,7 @@ describe('ACP same-session goal snapshot', () => {
     const calls = events.filter(event => event.type === 'tool/call').map(event => event.data.name)
     expect(calls).toEqual(['create_goal', 'get_goal'])
     const rounds = events.flatMap(event => event.type === 'user/message' && event.data.source.kind === 'goal'
+      && event.data.source.round > 0
       ? [event.data.source.round]
       : [])
     expect(rounds).toEqual([1, 2])