feat(v2): builder-agent self-modification WIP + container-config as per-group file

Checkpoints the builder-agent dev-agent/worktree/swap flow (create_dev_agent,
request_swap, classifier, deadman, promote) before pivoting to a unified
draft-activate approach with OS-level RO enforcement. Lifts container_config
out of the agent_groups row into groups/<folder>/container.json so install_packages,
add_mcp_server, and rebuild flows can eventually route through the same draft
path as source edits.

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

src/db/messaging-groups.ts

@@ -85,6 +85,20 @@ export function createMessagingGroupAgent(mga: MessagingGroupAgent): void {
 
   // Auto-create an agent_destinations row so delivery's ACL doesn't block
   // outbound messages that target this chat.
+  //
+  // ⚠️  DESTINATION PROJECTION NOTE: this function only writes the central
+  // `agent_destinations` row. It does NOT project into any running
+  // agent's session inbound.db (see top-of-file invariant in
+  // src/db/agent-destinations.ts). In practice this is fine because the
+  // only real callers are one-shot setup scripts (setup/register.ts,
+  // scripts/init-first-agent.ts, /manage-channels skill) that run in a
+  // separate process from the host. Any already-running container for
+  // `mga.agent_group_id` will keep serving the stale projection until
+  // its next wake (idle timeout or next inbound message) at which
+  // point spawnContainer's writeDestinations call refreshes from central.
+  // If you call this from code that runs INSIDE the host process and
+  // need the refresh to happen immediately, explicitly call
+  // `writeDestinations(mga.agent_group_id, <sessionId>)` afterwards.
   const existing = getDestinationByTarget(mga.agent_group_id, 'channel', mga.messaging_group_id);
   if (existing) return;