Add claude-code and gh CLI to Nix flake

[Copilot]

Adds claude-code and gh to the development shell's available tools.

Changes

• Added claude-code (Anthropic's Claude Code CLI) to buildInputs
• Added gh (GitHub CLI) to buildInputs
• Updated shellHook welcome message to list the new tools

Both tools are now available immediately when entering the dev environment via nix develop or direnv.

Original prompt

Add claude code and gh cli to the flake

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[github-actions]

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