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/tasks/tasks/tests/tasks.spec.ts

@@ -24,6 +24,7 @@ function stubAgent(ctx: Context, rawId: string): Agent {
     status: 'idle' as const,
     ctx: scopeFiber.ctx,
     send() {},
+    followup() {},
     steer() {},
     inject() {},
     cancel() {},