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/tui/tests/session-reference.snapshot.ts

@@ -128,7 +128,7 @@ describe('TUI session-reference snapshot', () => {
       type: 'text',
       text: '\n\n## My request:\n',
     })
-    expect(target.session.events.some(event => event.type === 'context/message')).toBe(false)
+    expect(target.session.events.some(event => event.type === 'user/message' && event.data.source.kind !== 'user')).toBe(false)
 
     const snapshot = await terminal.snapshot({ includeScrollback: true })
     if (REFRESHING) {