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/session-reference/tests/session-reference.spec.ts

@@ -61,7 +61,7 @@ function appendConversation(session: Session): void {
     { surfaceOp: 'append' },
   )
   session.append(
-    'context/message',
+    'user/message',
     { content: [{ type: 'text', text: 'workspace secret' }], source: { kind: 'plugin', plugin: 'workspace' } },
     { surfaceOp: 'append' },
   )