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/time-context/src/index.ts

@@ -64,7 +64,6 @@ function precedingMessageTime(agent: Agent): number | undefined {
       case 'user/message':
       case 'assistant/message':
       case 'tool/result':
-      case 'context/message':
       case 'steering/message':
         return event.time
       default:
@@ -79,7 +78,7 @@ function precedingMessageTime(agent: Agent): number | undefined {
 function precedingStepContextTime(agent: Agent, turn: number): number | undefined {
   for (const event of [...agent.session.events].reverse()) {
     if (event.type === 'turn/start' && event.data.turn === turn) return undefined
-    if (event.type === 'context/message'
+    if (event.type === 'user/message'
       && event.data.source.kind === 'plugin'
       && event.data.source.plugin === name) {
       return event.time
@@ -91,7 +90,7 @@ function precedingStepContextTime(agent: Agent, turn: number): number | undefine
 /** Find this plugin's latest durable injection, including a shadowed surface event. */
 function latestInjectionTime(agent: Agent): number | undefined {
   for (const event of [...agent.session.events].reverse()) {
-    if (event.type === 'context/message'
+    if (event.type === 'user/message'
       && event.data.source.kind === 'plugin'
       && event.data.source.plugin === name) {
       return event.time