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/core/session/tests/derived-cache.spec.ts

@@ -38,7 +38,7 @@ describe('derived-message cache', () => {
     expect(beforeReplace).toHaveLength(2)
 
     const nodes = session.surface.nodes
-    session.append('context/message', {
+    session.append('user/message', {
       content: [{ type: 'text', text: 'summary' }], source: { kind: 'plugin', plugin: 'compact' },
     }, { surfaceOp: { op: 'replace', start: nodes[0]!, end: nodes[1]! }, sourceEventSeqs: [nodes[0]!, nodes[1]!] })