feat(ralphex-fe): add RTK for token savings

[gatezh]

Closes #68

Summary

• Add rtk-download parallel build stage to ralphex-fe/Dockerfile — same pattern as .devcontainer/Dockerfile
• Copy RTK binary to /usr/local/bin/rtk in the final image
• Run rtk init -g --auto-patch in init-docker.sh as idempotent fallback for standalone usage (host mount normally provides the hook)
• Add rtk --version to CI verify step
• Add RTK to README tools table

Test plan

• CI hadolint passes on ralphex-fe/Dockerfile
• CI build + verify passes (rtk --version exits 0)
• Build locally on ARM64 — RTK binary downloads and runs
• Run container with host ~/.claude mount — RTK hook works via copied hooks
• Run container without mount — rtk init -g --auto-patch sets up the hook