feat: add Nix flake for reproducible builds

[gitmpr]

Summary

• Adds flake.nix and flake.lock so wt can be installed via the Nix package manager
• Users can install with nix profile install github:timvw/wt or run directly with nix run github:timvw/wt
• Version string is injected at build time via ldflags (-X github.com/timvw/wt/cmd.Version=<version>)

Install

# Run directly
nix run github:timvw/wt -- version

# Install to profile
nix profile install github:timvw/wt

Sandbox notes

The full test suite passes without skipping any tests. Three Nix build sandbox quirks required explicit handling:

• git, bash, util-linux in nativeCheckInputs — tests need git in PATH, a reachable bash binary, and script(1) (from util-linux), which the wt shell function uses to allocate a PTY for interactive commands
• postPatch shebang rewrite — TestRemoveCleansUpResidualWorktreeDirectory generates a #!/usr/bin/env bash wrapper script at runtime, but /usr/bin/env is absent in the Nix sandbox; the patch rewrites it to an absolute nix store path
• preCheck git init — the sandbox strips .git from the source tree; TestGetAvailableBranches and TestDefaultCmdRunsWithoutError call git in the working directory, so preCheck initialises a fresh repo

Test plan

• nix build .#wt passes (all tests, no -short flag)
• ./result/bin/wt version prints wt version 0.1.29

[timvw]

Thank you so much for this, @gitmpr! 🙏 Your flake was the foundation here — the sandbox workarounds (nativeCheckInputs, the git init in preCheck, the shebang handling) were spot on and did the hard part.

We folded your commits into #100 (preserved with a Co-authored-by on the squash) and layered on some follow-ups from a review pass: excluding the internal e2e main package so only wt is installed, deriving the version from the git rev to avoid drift, resolving bash via exec.LookPath (which let us drop the postPatch shebang rewrite entirely), bundling git in the runtime PATH, gating util-linux to Linux, and a Linux Nix CI job + Dockerfile so the flake stays tested.

#100 is merged, so I'll close this one — but the credit is yours. Really appreciate you adding Nix support! 🎉