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/tests/inbox.spec.ts

@@ -1,11 +1,20 @@
 import { describe, expect, it } from 'vitest'
 import { AgentMessageId } from '@deepseek-ai/dsh-agent'
-import { Inbox } from '../src/inbox.ts'
+import { Inbox, agentMessage } from '../src/inbox.ts'
 
 function message(text: string) {
   return { id: AgentMessageId(text), content: [{ type: 'text' as const, text }], source: { kind: 'user' as const }, contexts: [], wakeup: true }
 }
 
+describe('agentMessage', () => {
+  it('returns a frozen payload so a listener cannot mutate it for later listeners', () => {
+    const payload = agentMessage(message('m'), false)
+    expect(Object.isFrozen(payload)).toBe(true)
+    expect(() => { (payload as { id: string }).id = 'mutated' }).toThrow()
+    expect(payload.id).toBe(AgentMessageId('m'))
+  })
+})
+
 function resolverPair() {
   let r!: () => void
   const p = new Promise<void>((resolve) => { r = resolve })