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/ui/jsonrpc/tests/server.spec.ts

@@ -241,7 +241,7 @@ describe('HarnessSdkServer', () => {
           turn: 2,
           trigger: { kind: 'injection', source: { kind: 'plugin', plugin: 'late-metadata' } },
         })
-        session.append('context/message', {
+        session.append('user/message', {
           content: [{ type: 'text', text: 'late metadata' }],
           source: { kind: 'plugin', plugin: 'late-metadata' },
         }, { surfaceOp: 'append' })