feat(lib): add NonEmptyArray helper, tighten preferredRunner

[wyattjoh]

Summary

• Adds lib/helpers/arrays.ts with a NonEmptyArray<T> type alias and an isNonEmpty<T> type-guard predicate.
• Tightens preferredRunner to require a NonEmptyArray<Runner> input and return Runner (no longer | undefined).
• Updates the two call sites (init/skills.ts, init/format.ts) to use isNonEmpty for the non-empty check, and drops the now-provably-unreachable defensive if (!preferred) branch in skills.ts. The type system enforces the invariant that runtime code previously checked.

Stacked on #124.

Test plan

• bun run test passes (60 passed)
• bun test packages/cli-core/src/lib/helpers/arrays.test.ts covers the type guard

[wyattjoh]

Stack: init-skills

• feat(lib): add NonEmptyArray helper, tighten preferredRunner #125 👈 this PR
  • feat: Clerk CLI Skill #126
    • feat(init): support CLERK_SKILL_SOURCE env override #127
    • docs(skill): add audit-clerk-skill for drift detection #156

Part of a stacked PR chain. Do not merge manually.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 8293af40-5596-4c25-b0d2-d6af154d8da7

📥 Commits

Reviewing files that changed from the base of the PR and between 5ae8ed6 and f9c2e28.

📒 Files selected for processing (7)

• .changeset/fiery-comics-shave.md
• packages/cli-core/src/commands/init/format.ts
• packages/cli-core/src/commands/init/skills.ts
• packages/cli-core/src/lib/helpers/arrays.test.ts
• packages/cli-core/src/lib/helpers/arrays.ts
• packages/cli-core/src/lib/runners.test.ts
• packages/cli-core/src/lib/runners.ts

✅ Files skipped from review due to trivial changes (2)

• .changeset/fiery-comics-shave.md
• packages/cli-core/src/lib/helpers/arrays.test.ts

🚧 Files skipped from review as they are similar to previous changes (4)

• packages/cli-core/src/commands/init/format.ts
• packages/cli-core/src/lib/runners.ts
• packages/cli-core/src/lib/helpers/arrays.ts
• packages/cli-core/src/lib/runners.test.ts

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📝 Walkthrough

Walkthrough

Adds a NonEmptyArray<T> type and an isNonEmpty() type guard with tests. preferredRunner() signature changed to require available: NonEmptyArray<Runner> and to always return a Runner (removed empty-array handling). Call sites in init/format and init/skills now gate with isNonEmpty(available) and no longer treat preferredRunner() as possibly undefined. Runner tests were updated to use non-empty inputs and removed empty-list assertions. A new empty changeset file was added.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main changes: adding a NonEmptyArray helper utility and tightening the preferredRunner function signature.
Description check	✅ Passed	The description is directly related to the changeset, providing a clear summary of the new utilities added and the function signature changes made.

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[changeset-bot]

🦋 Changeset detected

Latest commit: f9c2e28

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