refactor: scaffold module registries and default-module layout

Additive change — existing code paths still run via inline fallbacks.
Prepares core for per-module extractions in PR #3 onward.

Four registries added with empty defaults:
  - delivery action handlers (delivery.ts)
  - router inbound gate (router.ts)
  - response dispatcher (index.ts)
  - MCP tool self-registration (container/agent-runner/src/mcp-tools/server.ts)

Default modules moved to src/modules/ for signaling:
  - src/modules/typing/       (extracted from delivery.ts)
  - src/modules/mount-security/ (moved from src/mount-security.ts)

Both are imported directly by core — no hook, no registry. Removal
requires editing core imports.

Migrator now keys applied rows by name (uniqueness) so module
migrations can pick arbitrary version numbers. Stored version column
is auto-assigned as an applied-order sequence.

sqlite_master guards added around core calls into module-owned tables
(user_roles, agent_destinations, pending_questions). No-ops today;
load-bearing after the owning modules are extracted.

MODULE-HOOK markers placed at scheduling's two skill-edit sites
(host-sweep.ts recurrence call, poll-loop.ts pre-task gate). PR #4
replaces the marked blocks when scheduling moves to its module.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/session-manager.ts

@@ -17,6 +17,7 @@ import path from 'path';
 import { DATA_DIR } from './config.js';
 import { getAgentGroup } from './db/agent-groups.js';
 import { getDestinations } from './db/agent-destinations.js';
+import { getDb, hasTable } from './db/connection.js';
 import { getMessagingGroup } from './db/messaging-groups.js';
 import { createSession, findSession, findSessionByAgentGroup, getSession, updateSession } from './db/sessions.js';
 import {
@@ -183,6 +184,11 @@ export function writeDestinations(agentGroupId: string, sessionId: string): void
   const dbPath = inboundDbPath(agentGroupId, sessionId);
   if (!fs.existsSync(dbPath)) return;
 
+  // Guarded: when the agent-to-agent module isn't installed, the
+  // `agent_destinations` table doesn't exist. Skip silently — core
+  // container spawn continues without projecting destinations.
+  if (!hasTable(getDb(), 'agent_destinations')) return;
+
   const rows = getDestinations(agentGroupId);
   const resolved: DestinationRow[] = [];