fix(agent-loop): address second-round review — disposal discard, injection validation, frozen payloads

Address the review bot's five genuinely-new findings on the current code:
- disposal now discards any still-pending inbox items before the loop
  exits, so every enqueued id gets a terminal lifecycle event.
- injection (next-step/no-wakeup) validates its payload up front, before
  opening the idle one-shot turn, honoring 'invalid input throws before
  any append'; and rejects attached contexts (which belong only to inbox
  messages) rather than silently dropping them.
- agentMessage() freezes the agent/inbox/* payload so a listener cannot
  mutate the shared correlation object mid-dispatch.
- refresh the package READMEs (compact, goal, guard, hook-protocol,
  plan-mode, time-context, workspace-context) that still referenced the
  removed context/message event, with the source-based user/message
  distinction.

The up-front injection validation makes two finally branches unreachable
(v8-ignored as the turn-enclosure backstop). Adds regression tests for
disposal discard, context rejection, up-front validation, and the frozen
payload; per-file coverage stays 100%.

packages/core/agent-loop/src/inbox.ts

@@ -29,7 +29,14 @@ export interface InboxMessage {
  * @returns the live-event message for enqueue/dequeue/discard.
  */
 export function agentMessage(message: InboxMessage, steering: boolean): AgentMessage {
-  return { id: message.id, content: message.content, source: message.source, contexts: message.contexts, steering, wakeup: message.wakeup }
+  // Frozen: the fused emitter passes this exact object to every listener in
+  // turn, so one listener must not be able to mutate a field (`id`, `steering`,
+  // `content`, …) a later listener then observes. `message` is already a frozen
+  // inbox record, so its nested fields need no re-clone.
+  return Object.freeze({
+    id: message.id, content: message.content, source: message.source,
+    contexts: message.contexts, steering, wakeup: message.wakeup,
+  })
 }
 
 /**