fix(agent-runner): require explicit destination addressing, fix per-destination threading

The poll loop had a bare-text routing fallback in dispatchResultText: when
the agent produced text without <message to="..."> wrapping, it would auto-
route to the session's originating channel (via a frozen RoutingContext) or
to the single configured destination. This caused three problems:

1. Routing drift: RoutingContext was extracted once from the initial batch
   and never refreshed. When the initial batch was a null-routed cron task
   and a real chat arrived mid-query, replies were silently dropped to
   scratchpad because the frozen routing had all-null fields.

2. Cross-channel thread bleed: sendToDestination applied a single
   routing.threadId to every outbound message regardless of destination.
   In agent-shared sessions (multiple channels sharing one session), one
   channel's thread ID was stamped onto messages to a different channel.

3. Inconsistent formatting: task, webhook, and system messages had no
   origin metadata in their formatted output, so the agent couldn't tell
   which destination they came from — even when the underlying messages_in
   rows carried routing fields.

Changes:

- Remove the bare-text routing fallbacks in dispatchResultText (both the
  routing-based and single-destination shortcuts). All agent output must
  be wrapped in <message to="name">...</message>. Bare text is scratchpad.

- Update buildDestinationsSection() to require explicit wrapping for all
  groups, including single-destination. No more "no special wrapping
  needed" shortcut.

- Resolve thread_id per-destination via resolveDestinationThread(), which
  queries messages_in for the most recent message matching the target
  channel+platform. Falls back to null (top-level channel message) when
  no prior inbound exists for that destination.

- Extract originAttr() helper in formatter.ts and apply it to all message
  types. Tasks now render as <task from="dest" time="...">, webhooks as
  <webhook from="dest" source="..." event="...">, system responses as
  <system_response from="dest" ...>. The agent always sees where a
  message originated.

- Add a PreCompact shell hook (compact-instructions.ts) that outputs
  custom compaction instructions, telling the compactor to preserve
  recent message XML structure and routing metadata in the summary.
  Wired via settings.json in the .claude-shared scaffold, with a
  migration path (ensurePreCompactHook) for existing groups.

Relation to open PRs:

- #2277 (mergeRouting) becomes unnecessary — the routing fallback it
  patches no longer exists. Can be closed.
- #2327 (post-compaction destination reminder) is complementary — it
  handles the post-compaction push, this handles pre-compaction
  instructions. Both can merge independently.
- #2328 (default routing instruction) is complementary — it adds "reply
  to the from= destination" guidance to the multi-destination section.
  Compatible with the unified instruction format here.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/poll-loop.test.ts

@@ -47,7 +47,7 @@ describe('formatter', () => {
     insertMessage('m1', 'task', { prompt: 'Review open PRs' });
     const messages = getPendingMessages();
     const prompt = formatMessages(messages);
-    expect(prompt).toContain('[SCHEDULED TASK]');
+    expect(prompt).toContain('<task');
     expect(prompt).toContain('Review open PRs');
   });
 
@@ -55,15 +55,17 @@ describe('formatter', () => {
     insertMessage('m1', 'webhook', { source: 'github', event: 'push', payload: { ref: 'main' } });
     const messages = getPendingMessages();
     const prompt = formatMessages(messages);
-    expect(prompt).toContain('[WEBHOOK: github/push]');
+    expect(prompt).toContain('<webhook');
+    expect(prompt).toContain('source="github"');
+    expect(prompt).toContain('event="push"');
   });
 
   it('should format system messages', () => {
     insertMessage('m1', 'system', { action: 'register_group', status: 'success', result: { id: 'ag-1' } });
     const messages = getPendingMessages();
     const prompt = formatMessages(messages);
-    expect(prompt).toContain('[SYSTEM RESPONSE]');
-    expect(prompt).toContain('register_group');
+    expect(prompt).toContain('<system_response');
+    expect(prompt).toContain('action="register_group"');
   });
 
   it('should handle mixed kinds', () => {
@@ -72,7 +74,7 @@ describe('formatter', () => {
     const messages = getPendingMessages();
     const prompt = formatMessages(messages);
     expect(prompt).toContain('sender="John"');
-    expect(prompt).toContain('[SYSTEM RESPONSE]');
+    expect(prompt).toContain('<system_response');
   });
 
   it('should escape XML in content', () => {