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-doc-graphs.ts

@@ -911,7 +911,7 @@ function renderLifecycle(): string {
     '  participant Persistence',
     '  participant SDK as UI or SDK listener',
     '  User->>Agent: send(content)',
-    `  Agent-->>SDK: ${mermaidCode('agent/queued')}`,
+    `  Agent-->>SDK: ${mermaidCode('agent/inbox/enqueue')}`,
     '  Agent->>Driver: queued work wakes driver',
     `  Driver-->>SDK: ${mermaidCode('agent/status')} running`,
     `  Driver->>Session: ${mermaidCode('turn/start')}`,
@@ -988,7 +988,7 @@ function renderToolPipeline(): string {
     `  owned["Tool-owned session events<br/>${mermaidCode('todo/write')}, ${mermaidCode('fs/observed')}, ${mermaidCode('hook/invoked')}, ${mermaidCode('hook/result')}, ${mermaidCode('tool/code-dispatch')}"]`,
     `  post["${mermaidCode('tools/post-execute')} waterfall<br/>accept, block, replace, add context"]`,
     `  final["${mermaidCode('tools/result')} synchronous notification<br/>frozen authoritative outcome"]`,
-    '  context["Active-batch additionalContexts FIFO<br/>context/message after recorded tool results"]',
+    '  context["Active-batch additionalContexts FIFO<br/>injected user/message after recorded tool results"]',
     `  toolResult["Session event: ${mermaidCode('tool/result')}<br/>single model-facing outcome"]`,
     '  allResults["Tool batch settled<br/>recorded tool/result events complete"]',
     '  presentResult["UI completed card<br/>presentResult(args, result)"]',