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/context/time-context/tests/time-context.e2e.ts

@@ -48,7 +48,7 @@ describe('time-context through a real headless cordis.yml', () => {
     expect(stderr).not.toContain('UNHANDLED')
     expect(events.filter(event => event.type === 'turn/end')).toHaveLength(2)
 
-    const contexts = events.filter(event => event.type === 'context/message')
+    const contexts = events.filter(event => event.type === 'user/message')
     const starts = events.filter(event => event.type === 'step/start')
     expect(contexts).toHaveLength(2)
     expect(starts).toHaveLength(2)