refactor(v2): remove builder-agent dev-agent/worktree/swap flow

The dev-agent-in-worktree approach for source self-modification is abandoned
in favor of a direct draft/activate flow with OS-level RO enforcement
(planned, not yet implemented). Strip the whole subgraph:
src/builder-agent/, pending-swaps DB module + migration 006, builder-agent
MCP tools, and all host wiring (startup sweep, approval routing, deadman,
worktree mount, freeze gate). Tool descriptions in self-mod.ts / agents.ts
no longer cross-reference create_dev_agent. CLAUDE.md + v2-checklist updated
to describe the new direction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/delivery.ts

@@ -851,32 +851,6 @@ async function handleSystemAction(
       break;
     }
 
-    case 'create_dev_agent': {
-      const { handleCreateDevAgent } = await import('./builder-agent/handlers.js');
-      await handleCreateDevAgent(
-        {
-          requestId: content.requestId as string,
-          name: content.name as string,
-        },
-        session,
-        notifyAgent,
-      );
-      break;
-    }
-
-    case 'request_swap': {
-      const { handleRequestSwap } = await import('./builder-agent/handlers.js');
-      await handleRequestSwap(
-        {
-          perFileSummaries: (content.perFileSummaries as Record<string, string>) || {},
-          overallSummary: (content.overallSummary as string) || '',
-        },
-        session,
-        notifyAgent,
-      );
-      break;
-    }
-
     default:
       log.warn('Unknown system action', { action });
   }