Expose host-native CLI binary as the default flake package

[negz]

Description of your changes

Fixes #125

The default package was the full release bundle: the CLI cross-compiled for all seven platforms, laid out under bin/<os>_<arch>/crossplane. This made nix build and nix run unhelpful for anyone consuming the CLI from another flake. nix run couldn't find a binary to run, and producing one binary meant a from-source build of all seven, six of which you'd throw away.

This PR makes default the host-native binary, built for the system Nix is running on, so nix build and nix run give you a single binary for your own machine. The multi-platform bundle moves to a new package named release.

The per-platform builder already existed to assemble the bundle, so default just wraps it for the host, deriving GOOS and GOARCH from stdenv.hostPlatform. The CI build-artifacts job now builds .#release explicitly so it still produces and uploads the full bundle. The push-artifacts app already referenced the release derivation directly, so the S3 push and promote flows are unaffected.

I have:

• Read and followed Crossplane's contribution process.
• Run ./nix.sh flake check to ensure this PR is ready for review.
• Added or updated unit tests.
• Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Pro

Run ID: 366db893-5eac-4c04-b0d2-2ca2aff87aab

📥 Commits

Reviewing files that changed from the base of the PR and between 62cadf8 and 5466a88.

⛔ Files ignored due to path filters (1)

• nix/build.nix is excluded by none and included by none

📒 Files selected for processing (2)

• .github/workflows/ci.yml
• flake.nix

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📝 Walkthrough

Walkthrough

flake.nix reassigns packages.<system>.default to build.binary (host-native binary) and adds a new packages.<system>.release attribute for the multi-platform bundle via build.release. The CI workflow's "Build Artifacts" step is updated to explicitly target .#release instead of the implicit default.

Changes

Flake default/release split and CI update

Layer / File(s)	Summary
Split flake packages.default / packages.release and wire CI flake.nix, .github/workflows/ci.yml	packages.<system>.default is repointed from build.release to build.binary { inherit version; }, a new packages.<system>.release attribute is defined using build.release { inherit version goPlatforms; }, and the CI "Build Artifacts" step is updated to invoke nix build .#release to consume the renamed bundle target.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Title check	✅ Passed	The PR title clearly and accurately describes the main change: exposing the host-native CLI binary as the default flake package, meeting the descriptive requirement under 72 characters.
Description check	✅ Passed	The PR description is directly related to the changeset, providing thorough context about the motivation, implementation details, and affected CI workflows.
Linked Issues check	✅ Passed	The PR fully addresses the objectives from issue #125: exposes host-native binary as default, moves multi-platform bundle to release package, and updates CI to build .#release explicitly.
Out of Scope Changes check	✅ Passed	All changes are directly related to the objectives: updating flake.nix to restructure packages and modifying CI to reference the new release package explicitly.
Breaking Changes	✅ Passed	This PR only modifies flake.nix and .github/workflows/ci.yml for Nix build configuration. No changes to files under apis/** or cmd/** remove/rename public fields/flags, add required fields/flags, o...
Feature Gate Requirement	✅ Passed	PR only changes Nix flake package structure (default→binary, added release→bundle); no APIs affected, no experimental features added.

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