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.

examples/headless-agent/tests/code-mode.e2e.ts

@@ -151,7 +151,8 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('Code Mode: real model writes a p
     const events: SessionEvent[] = [...handle.agent.session.events]
     const dispatch = events.find(event => event.type === 'tool/code-dispatch' && event.data.name === 'read')
     const outerResult = events.find(event => event.type === 'tool/result')
-    const workspaceContext = events.find(event => event.type === 'context/message'
+    const workspaceContext = events.find(event => event.type === 'user/message'
+      && event.data.source.kind === 'plugin'
       && typeof event.data.meta === 'object'
       && event.data.meta !== null
       && !Array.isArray(event.data.meta)