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/schema.ts

@@ -93,11 +93,13 @@ CREATE TABLE messages_in (
   content        TEXT NOT NULL
 );
 
--- Host tracks which messages_out IDs have been delivered.
+-- Host tracks delivery outcomes for messages_out IDs.
 -- Avoids writing to outbound.db (container-owned).
 CREATE TABLE delivered (
-  message_out_id TEXT PRIMARY KEY,
-  delivered_at   TEXT NOT NULL
+  message_out_id      TEXT PRIMARY KEY,
+  platform_message_id TEXT,
+  status              TEXT NOT NULL DEFAULT 'delivered',
+  delivered_at        TEXT NOT NULL
 );
 `;