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/db/connection.ts

@@ -31,3 +31,18 @@ export function closeDb(): void {
   _db?.close();
   _db = null;
 }
+
+/**
+ * Check whether a table exists. Used by core code that touches
+ * module-owned tables so that an uninstalled module degrades silently
+ * instead of raising SQLite errors. Cheap: a single indexed lookup on
+ * sqlite_master. Results are not cached — a module install adds the
+ * table at runtime (next service start), and callers may run before
+ * or after that boundary.
+ */
+export function hasTable(db: Database.Database, name: string): boolean {
+  const row = db.prepare(`SELECT 1 FROM sqlite_master WHERE type='table' AND name = ? LIMIT 1`).get(name) as
+    | { '1': number }
+    | undefined;
+  return row !== undefined;
+}