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/pty-local/tests/index.spec.ts

@@ -42,7 +42,7 @@ function agent(ctx: Context): Agent {
   const id = SessionId('agent')
   return {
     id, options: {}, session: new Session(id), status: 'idle', ctx,
-    send() {}, steer() {}, inject() {}, cancel() {}, whenIdle: () => Promise.resolve(),
+    send() {}, followup() {}, steer() {}, inject() {}, cancel() {}, whenIdle: () => Promise.resolve(),
   }
 }
 

packages/pty/pty-local/tests/local.spec.ts

@@ -34,7 +34,7 @@ function stubAgent(ctx: Context, rawId: string): Agent {
   const scope = ctx.plugin(() => {})
   return {
     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(),
   }
 }