test: add integration tests for the shipped action

[EndBug]

Summary

• Add Jest integration tests that spawn lib/index.js against temporary git fixtures with local bare remotes (covers commit, tag, push, new branch, remove, selective add).
• Keep fixtures under os.tmpdir() with an allowlisted spawn env so CI never pushes to GitHub or touches the checked-out repo.
• Update CONTRIBUTING to run npm test instead of manually testing in a separate repo.

Closes #479

Test plan

• npm test (unit + integration + lint) passes locally
• CI Test workflow is green on this PR
• Spot-check that push scenarios only hit the local bare remote (no network)

Made with Cursor

Summary by CodeRabbit

• Tests

  • Added comprehensive integration coverage for committing, tagging, pushing, branching, author settings, and selective file changes.
  • Added isolated Git test fixtures and validation helpers to improve reliability of action testing.

• Documentation

  • Updated contribution instructions to require running tests after building, including unit and integration tests.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 8d69ee7d-e12a-4313-a324-7c5e1a2c1b00

📥 Commits

Reviewing files that changed from the base of the PR and between 75ed20e and 360a98f.

📒 Files selected for processing (1)

• test/integration/helpers.ts

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📝 Walkthrough

Walkthrough

Adds isolated Git fixtures and helpers for running the built action. Adds integration tests for commits, no-ops, identities, tags, pushes, branches, and pathspec behavior. Updates contribution instructions to run unit and integration tests after building.

Changes

Integration testing

Layer / File(s)	Summary
Fixture and action harness test/integration/helpers.ts	Creates temporary Git repositories, runs the built action with controlled inputs, parses outputs, and provides Git inspection helpers.
Action behavior integration coverage test/integration/action.test.ts, CONTRIBUTING.md	Tests commit, no-op, identity, tag, push, branch, removal, and selective-addition behavior. Updates the required test command.

Estimated code review effort: 3 (Moderate) | ~30 minutes

🚥 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 identifies the primary change: adding integration tests for the shipped action.
Linked Issues check	✅ Passed	The changes satisfy issue #479 by testing commit creation and push behavior with isolated local Git fixtures.
Out of Scope Changes check	✅ Passed	All changes support the testing objective, including test documentation, fixtures, helpers, and action integration coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cursor/add-integration-tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/integration/helpers.ts`:
- Around line 61-80: Update the default-branch lookup in the fixture setup to
handle a missing init.defaultBranch without letting git config failure abort
execution, using the intended fallback branch. Ensure the seed checkout and push
use that resolved branch, then explicitly set the bare remote’s HEAD to it after
the push so the remote default matches the fixture branch.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 9519cf21-2810-4633-b120-97cc22511562

📥 Commits

Reviewing files that changed from the base of the PR and between df70d11 and 75ed20e.

📒 Files selected for processing (3)

• CONTRIBUTING.md
• test/integration/action.test.ts
• test/integration/helpers.ts