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>

container/agent-runner/src/mcp-tools/agents.ts

@@ -31,7 +31,7 @@ export const createAgent: McpToolDefinition = {
   tool: {
     name: 'create_agent',
     description:
-      "Create a long-lived companion sub-agent (research assistant, task manager, specialist) — the name becomes your destination for it. NOT for source-code changes — use `create_dev_agent` for those. Admin-only. Fire-and-forget.",
+      'Create a long-lived companion sub-agent (research assistant, task manager, specialist) — the name becomes your destination for it. Admin-only. Fire-and-forget.',
     inputSchema: {
       type: 'object' as const,
       properties: {