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/goal/goal-session/src/index.ts

@@ -306,10 +306,10 @@ export function apply(ctx: Context): void {
         requestDrive(state)
       }
     })
-    ctx.on('agent/queued', (agent, content, info) => {
+    ctx.on('agent/inbox/enqueue', (agent, info) => {
       const state = stateFor(agent)
       const attempt = state.attempt
-      if (attempt !== undefined && sameQueued(content, info.source, attempt)) return
+      if (attempt !== undefined && sameQueued(info.content, info.source, attempt)) return
       state.competingQueued = true
       if (attempt?.phase === 'queued') attempt.stale = true
     })