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.

scripts/gen-cordis-catalog.ts

@@ -36,6 +36,7 @@ export const LINK_MAP: Record<string, string> = {
   ContinuationStop: 'core.md',
   GenerateOptions: 'core.md',
   HookContext: 'core.md',
+  InboxItemInfo: 'core.md',
   LlmCallConfig: 'core.md',
   LlmModelContext: 'core.md',
   LlmFailure: 'llm-streaming.md',