feat: add nix flake packaging

[Microck]

what changed

adds first-class nix packaging for kagi-cli:

• adds a reproducible flake.nix and flake.lock
• exposes packages.default, apps.default, checks.kagi, devShells.default, and overlays.default
• packages the existing Cargo project with rustPlatform.buildRustPackage
• installs bash, zsh, and fish completions from the built binary
• documents nix run and nix profile install

why

kagi-cli is already a good fit for nix: it is a locked Rust project that builds a standalone binary and uses rustls instead of OpenSSL. this gives NixOS and home-manager users a declarative install path without changing the existing installers or release workflow.

verification

• cargo fmt --check
• cargo check --workspace --all-targets --locked
• cargo test --workspace --all-targets --locked
• 273 tests passed, 13 live tests ignored

nix flake check was not run because nix is not installed in the verification environment.

implementation with Hermes, verified locally with the existing Rust toolchain.

Summary by CodeRabbit

• New Features

  • Added Nix/NixOS installation support, including direct execution, profile installation, and flake integration.
  • Added a Nix package, runnable app, development shell, formatter, overlay, and package checks.

• Documentation

  • Added setup instructions for using the application with Nix, NixOS, and home-manager.

Greptile Summary

Adds first-class Nix flake packaging for the Rust CLI.

• Exposes default package, app, check, development shell, formatter, and overlay outputs.
• Builds the existing Cargo project and generates shell completions when the build platform can execute target binaries.
• Documents Nix execution, profile installation, NixOS, and Home Manager usage.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the prior cross-build issue is fixed by skipping target test execution when the build platform cannot execute host-platform binaries.

Important Files Changed

Filename	Overview
flake.nix	Defines per-system package, app, check, development shell, formatter, and exported overlay outputs.
nix/kagi.nix	Packages the Cargo project, correctly guards tests and completion generation by target executability, and defines package metadata.
flake.lock	Pins the nixpkgs, flake-utils, and systems inputs used by the new flake.
README.md	Adds concise Nix run, profile installation, NixOS, and Home Manager guidance.
docs/guides/installation.mdx	Adds Nix installation and authentication instructions to the installation guide.

_{Reviews (3): Last reviewed commit: "docs: add nix installation guide" | Re-trigger Greptile}

[coderabbitai]

📝 Walkthrough

Walkthrough

The pull request adds Nix packaging for the Rust CLI. It defines a Nix derivation, exposes flake outputs for building and development, and documents Nix, NixOS, and home-manager installation methods.

Changes

Nix packaging

Layer / File(s)	Summary
Rust package definition nix/kagi.nix	The derivation builds kagi from Cargo metadata and locked dependencies. It runs tests, installs Bash, Zsh, and Fish completions, and defines package metadata.
Flake outputs and installation flake.nix, README.md	The flake adds package, application, check, development shell, formatter, and overlay outputs. The README documents direct execution, profile installation, and NixOS or home-manager integration.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Release Metadata Consistency	✅ Passed	Cargo.toml, Cargo.lock kagi, and npm/package.json all contain 0.16.0; CHANGELOG.md contains the exact heading ## [0.16.0].
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding Nix flake packaging.
Description check	✅ Passed	The description explains the changes, rationale, verification results, and unavailable Nix verification; it omits some template sections but remains mostly complete.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/add-nix-flake

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Dismissed because a newer commit was pushed; Greptile will re-review the current head.