feat: agent-to-agent communication, dynamic agent creation, self-modification tools

Agent-to-agent: host routes messages with channel_type='agent' to target
agent's inbound.db, enriches with sender info, wakes target container.
Bidirectional routing works via inherited routing context.

Dynamic agents: create_agent MCP tool + system action handler creates
agent groups, folders, and optional CLAUDE.md on the fly.

Self-modification: install_packages (apt/npm, requires admin approval),
add_mcp_server (no approval), request_rebuild (builds per-agent-group
Docker image with approved packages). Approval flow reuses interactive
card infrastructure with pending_approvals table.

Also includes fixes from prior session: attachment download, reply context
extraction, message editing (platform message ID tracking), delivery retry
limits, and card update on button click.

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

src/db/messaging-groups.ts

@@ -109,3 +109,14 @@ export function updateMessagingGroupAgent(
 export function deleteMessagingGroupAgent(id: string): void {
   getDb().prepare('DELETE FROM messaging_group_agents WHERE id = ?').run(id);
 }
+
+/** Get all messaging groups wired to an agent group (reverse lookup). */
+export function getMessagingGroupsByAgentGroup(agentGroupId: string): MessagingGroup[] {
+  return getDb()
+    .prepare(
+      `SELECT mg.* FROM messaging_groups mg
+       JOIN messaging_group_agents mga ON mga.messaging_group_id = mg.id
+       WHERE mga.agent_group_id = ?`,
+    )
+    .all(agentGroupId) as MessagingGroup[];
+}