flake: skip npm ci in default shell when lockfile already installed

[thedavidmeister]

Summary

• The default devShell ran npm ci --ignore-scripts on every shell entry whenever a package.json was present, wiping and reinstalling node_modules even when it already matched the lockfile. Painful for ad-hoc nix develop -c <cmd> invocations.
• Compare ./package-lock.json to the copy npm writes into ./node_modules/.package-lock.json after a successful install, and skip npm ci when they match.
• First entry (or any entry after the lockfile changes) still runs npm ci.

Closes #175

Test plan

• nix develop in a repo with a package.json + package-lock.json triggers npm ci on first entry, then is a no-op on subsequent entries.
• After modifying package-lock.json, the next nix develop entry re-runs npm ci.
• nix develop in a repo without package.json remains a no-op.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Optimized development environment setup to run dependency installation only when necessary, improving development workflow efficiency.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b1844be5-09e5-47f8-84b0-50dc8a62135d

📥 Commits

Reviewing files that changed from the base of the PR and between b947843 and 8904a6e.

📒 Files selected for processing (1)

• flake.nix

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📝 Walkthrough

Walkthrough

The default devShell's shellHook in flake.nix now checks whether package-lock.json matches node_modules/.package-lock.json before running npm ci. This prevents unnecessary dependency reinstalls when the lockfile is already synchronized with the installed tree.

Changes

Dev Shell Optimization

Layer / File(s)	Summary
npm ci Guard Condition flake.nix	The shellHook condition is tightened to check both package.json existence and lockfile synchronization before running npm ci, skipping reinstalls when package-lock.json matches node_modules/.package-lock.json.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A single guard stands tall and wise,
Comparing locks with careful eyes—
No needless npm reinstalls today,
Shell entries faster than yesterday! 🚀

🚥 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 accurately and concisely describes the main change: skipping npm ci when the lockfile is already installed, matching the core objective.
Linked Issues check	✅ Passed	The code change directly implements the proposal from issue #175: comparing package-lock.json to node_modules/.package-lock.json to skip npm ci when they match.
Out of Scope Changes check	✅ Passed	The change is narrowly scoped to the intended modification of the npm ci condition in flake.nix, with no unrelated alterations.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-05-19-skip-npm-ci-when-lockfile-unchanged

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