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/modules/index.ts

@@ -0,0 +1,16 @@
+/**
+ * Modules barrel.
+ *
+ * Each module self-registers at import time. This barrel is imported by
+ * src/index.ts for side effects (registry registrations, typing impl setup,
+ * etc.). Core runs with an empty barrel — the registries have inline
+ * fallbacks and `sqlite_master` guards.
+ *
+ * Default modules (ship with main, direct core import):
+ *   - src/modules/typing/        → imported directly by router/delivery/container-runner
+ *   - src/modules/mount-security/ → imported directly by container-runner
+ *
+ * Registry-based modules (installed via /add-<name> skills, pulled from the
+ * `modules` branch): append imports below.
+ */
+export {};