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/db/migrations/index.ts

@@ -6,7 +6,6 @@ import { migration002 } from './002-chat-sdk-state.js';
 import { migration003 } from './003-pending-approvals.js';
 import { migration004 } from './004-agent-destinations.js';
 import { migration005 } from './005-pending-credentials.js';
-import { migration006 } from './006-pending-swaps.js';
 import { migration007 } from './007-pending-approvals-title-options.js';
 
 export interface Migration {
@@ -21,7 +20,6 @@ const migrations: Migration[] = [
   migration003,
   migration004,
   migration005,
-  migration006,
   migration007,
 ];