feat: add Codex CLI support, multi-agent build toggles

[n1mmy]

Summary

Transforms the container from Claude-only to a multi-agent coding environment.

New capabilities:

• Install OpenAI Codex CLI alongside Claude Code (both toggleable via build args)
• INSTALL_CLAUDE and INSTALL_CODEX build args to independently enable/disable each CLI
• EXTRA_APT_PACKAGES and EXTRA_NPM_PACKAGES build args for custom package installation
• OPENAI_API_KEY environment variable support (entrypoint injection + SSH AcceptEnv)

Renamed:

• EXTRA_PACKAGES → EXTRA_APT_PACKAGES (breaking change for existing builds using this arg)

Infrastructure:

• k8s manifest updated: secret renamed to ai-credentials, deployment/service renamed to agentic-coding-server, OPENAI_API_KEY env var added
• README fully rewritten for multi-agent framing
• .gitignore added for .claude/worktrees
• Fixed manifests.yaml typo → k8s-manifest.yaml in GPU docs

Pre-Landing Review

No issues found.

Test plan

• docker build . succeeds with defaults (both CLIs installed)
• docker build --build-arg INSTALL_CODEX=false . produces image without codex
• docker build --build-arg INSTALL_CLAUDE=false . produces image without claude
• Container starts and accepts SSH connections
• OPENAI_API_KEY propagates to ubuntu user's shell environment
• k8s manifest applies cleanly with kubectl apply -f k8s-manifest.yaml

🤖 Generated with Claude Code