ci: adopt unmango/actions

[UnstoppableMango]

Moves the Nix setup and release-please onto the shared unmango/actions v1.0.1.

What moved

• .github/actions/nix calls unmango/actions/setup-nix in place of the cachix/install-nix-action and cachix/cachix-action pair.
• The composite's inputs are unchanged, so no workflow call site changes.
• skipPush is dropped because cachix-action only pushes when it has an auth token, so an empty cachix-token still means read-only.
• release-please.yml calls the reusable unmango/actions/.github/workflows/release-please.yml with the same config and manifest files and RELEASE_PLEASE_TOKEN passed as the token secret.

What stayed inline

• Nothing.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Streamlined Nix setup and build caching for more consistent automated builds.
  • Improved handling of optional cache credentials during automated workflows.
  • Updated release automation to use a shared, version-pinned workflow for more consistent release preparation.
  • Simplified release workflow configuration while preserving existing release inputs and authentication.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: fa6d18ee-8f85-44df-a3bb-af5c52ea3514

📥 Commits

Reviewing files that changed from the base of the PR and between 81e9f0e and efbb63b.

📒 Files selected for processing (2)

• .github/actions/nix/action.yml
• .github/workflows/release-please.yml

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

The pull request replaces separate Nix and Cachix setup steps with a pinned composite action. It also replaces the inline release-please action with a pinned reusable workflow.

Changes

CI action updates

Layer / File(s)	Summary
Pinned Nix and cache setup .github/actions/nix/action.yml	The action uses the pinned unmango/actions/setup-nix action with GitHub and Cachix tokens. It removes the explicit cache name and skipPush logic.
Reusable release-please workflow .github/workflows/release-please.yml	The job calls the pinned reusable release-please workflow. It passes config-file and manifest-file as inputs and passes RELEASE_PLEASE_TOKEN as a secret.

Priority: ⬇️ Low

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

Change: Other

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adopting the shared unmango/actions CI configuration.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

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🧪 Generate unit tests (beta)

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