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/src/index.ts

@@ -332,7 +332,7 @@ export class PlanModeService extends Service {
     const text = target
       ? 'The user switched this session to plan mode.'
       : 'The user switched this session back to the default mode.'
-    session.append('context/message', {
+    session.append('user/message', {
       content: [{ type: 'text', text }],
       source: { kind: 'plugin', plugin: 'plan-mode' },
     }, { surfaceOp: 'append' })