Add ModCDP code mode to new evals

[pirate]

Summary

Adds a separate modcdp_code core tool surface for evals, alongside the existing cdp_code surface.

Changes

• Registers modcdp_code in the core tool surface union, registry, default startup profile resolver, CLI help, and architecture diagram.
• Adds ModCdpCodeTool, backed by the built ModCDP JS client from the same v4 checkout root used by STAGEHAND_V4_SDK_PATH, under modcdp/dist/client/js/ModCDPClient.js; MODCDP_CLIENT_PATH can still override it directly.
• Reuses the existing CDP page/session implementation through CdpConnectionLike so the ModCDP client can drive the same core browser operations.
• Wires Claude Code external-agent runs to modcdp_code, exposing a modcdp runtime object.
• Adds the four ModCDP primitives to the Claude Code system prompt append: Mod.evaluate, Mod.addCustomCommand, Mod.addCustomEvent, and Mod.addMiddleware.

Validation

• pnpm --filter @browserbasehq/stagehand-evals run typecheck
• pnpm --filter @browserbasehq/stagehand-evals run test:unit