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/pty/tool-pty/tests/loader-composition.spec.ts

@@ -40,7 +40,7 @@ function agent(ctx: Context): Agent {
   const id = SessionId('pty-loader-agent')
   const value: Agent = {
     id, options: {}, session: new Session(id), status: 'idle', ctx: scope.ctx,
-    send() {}, steer() {}, inject() {}, cancel() {}, whenIdle: () => Promise.resolve(),
+    send() {}, followup() {}, steer() {}, inject() {}, cancel() {}, whenIdle: () => Promise.resolve(),
   }
   ctx.agents.register(value)
   return value