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/host-sweep.ts

@@ -100,8 +100,14 @@ async function sweepSession(session: Session): Promise<void> {
       detectStaleContainers(inDb, outDb, session, agentGroup.id);
     }
 
-    // 4. Handle recurrence for completed messages
+    // 4. Handle recurrence for completed messages.
+    // MODULE-HOOK:scheduling-recurrence:start
+    // When scheduling is extracted (PR #4), `handleRecurrence` moves to
+    // `src/modules/scheduling/` and the `/add-scheduling` skill replaces
+    // this block with a call to the module. Without scheduling
+    // installed, the block is empty and recurrence is a no-op.
     await handleRecurrence(inDb, session);
+    // MODULE-HOOK:scheduling-recurrence:end
   } finally {
     inDb.close();
     outDb?.close();