Add Nix flake and fish shell completions

[schickling-assistant]

Summary

• Add flake.nix with buildNpmPackage derivation for the pty CLI
• Add fish shell completions (completions/pty.fish) covering all subcommands and flags
• Install bash, zsh, and fish completions to standard Nix paths (share/bash-completion/, share/zsh/site-functions/, share/fish/vendor_completions.d/)
• Add result to .gitignore for nix build output

Rationale

This makes pty installable via Nix flakes, which enables reproducible builds and easy consumption as a flake input in other projects:

# As a flake input
inputs.pty.url = "github:myobie/pty";

# Direct usage
nix run github:myobie/pty -- --help
nix shell github:myobie/pty

The derivation uses buildNpmPackage (nixpkgs) which handles the node-pty native prebuilds correctly and patches the shebang to use a Nix-provided Node.js.

Test plan

• nix build .#pty succeeds
• pty --help works from the built output
• pty run -d test sleep 5 && pty list && pty kill test — full session lifecycle works
• Bash completions produce correct subcommands
• Zsh completions installed at correct path
• Fish completions produce correct subcommands and flags

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Note

This PR was created on behalf of @schickling

🤖 Generated with Claude Code

[myobie]

@schickling-assistant thanks for this, I've now got something in main inspired from this PR and are grateful for your help 🫡