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/destinations.ts

@@ -102,28 +102,20 @@ function buildDestinationsSection(): string {
     ].join('\n');
   }
 
-  // Single-destination shortcut: the agent just writes its response normally.
+  const lines = ['## Sending messages', ''];
   if (all.length === 1) {
     const d = all[0];
     const label = d.displayName && d.displayName !== d.name ? ` (${d.displayName})` : '';
-    return [
-      '## Sending messages',
-      '',
-      `Your messages are delivered to \`${d.name}\`${label}. Just write your response directly — no special wrapping needed.`,
-      '',
-      'To mark something as scratchpad (logged but not sent), wrap it in `<internal>...</internal>`.',
-      '',
-      'To send a message mid-response (e.g., an acknowledgment before a long task), call the `send_message` MCP tool.',
-    ].join('\n');
-  }
-
-  const lines = ['## Sending messages', '', 'You can send messages to the following destinations:', ''];
-  for (const d of all) {
-    const label = d.displayName && d.displayName !== d.name ? ` (${d.displayName})` : '';
-    lines.push(`- \`${d.name}\`${label}`);
+    lines.push(`Your destination is \`${d.name}\`${label}.`);
+  } else {
+    lines.push('You can send messages to the following destinations:', '');
+    for (const d of all) {
+      const label = d.displayName && d.displayName !== d.name ? ` (${d.displayName})` : '';
+      lines.push(`- \`${d.name}\`${label}`);
+    }
   }
   lines.push('');
-  lines.push('To send a message, wrap it in a `<message to="name">...</message>` block.');
+  lines.push('**Every response must be wrapped** in a `<message to="name">...</message>` block.');
   lines.push('You can include multiple `<message>` blocks in one response to send to multiple destinations.');
   lines.push('Text outside of `<message>` blocks is scratchpad — logged but not sent anywhere.');
   lines.push('Use `<internal>...</internal>` to make scratchpad intent explicit.');