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/src/projection.ts

@@ -57,7 +57,6 @@ function projectSessionConversation(snapshot: SessionSurfaceSnapshot): Projected
         break
       }
       case 'tool/result':
-      case 'context/message':
         break
       /* v8 ignore next 2 -- SurfaceEventType is closed and every variant is handled above. */
       default: