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/plan/plan-mode/tests/plan-mode.spec.ts

@@ -95,7 +95,7 @@ function header(session: Session): void {
 
 function noticeTexts(session: Session): string[] {
   return session.events
-    .filter(event => event.type === 'context/message')
+    .filter(event => event.type === 'user/message' && event.data.source.kind === 'plugin')
     .map(event => (event.data as { content: { type: string; text?: string }[] }).content.map(block => block.text ?? '').join(''))
 }