fix(ci): lint only PR title since repo is squash-merge only

[cv]

Summary

The repo uses squash-merge exclusively. Individual commit messages are
discarded at merge time — only the PR title becomes the commit on main
and drives changelog generation.

This PR removes the per-commit lint step from the commit-lint workflow
and keeps only the PR title check. Also drops fetch-depth: 0 (no
longer needed) and fixes a pre-existing markdownlint MD032 in README.md.

Context: PR #913 has a commit created via GitHub's web-UI
"Apply suggestion" feature which doesn't follow conventional commit
format. Rather than special-casing those, this aligns the CI check with
what actually matters for the squash-merge workflow.

Test plan

• commit-lint CI job passes on this PR
• PR chore: add tsconfig.cli.json, root execa, TS coverage ratchet #913 commit-lint job passes after this merges

Summary by CodeRabbit

• Chores
  • Updated commit linting workflow to validate pull request titles.
  • Minor documentation formatting updates.

[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: 1dae80c2-05e7-40d4-9bea-a922c31d76b4

📥 Commits

Reviewing files that changed from the base of the PR and between 0acb2e4 and a7c999e.

📒 Files selected for processing (2)

• .github/workflows/commit-lint.yaml
• README.md

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📝 Walkthrough

Walkthrough

The commit-lint workflow was simplified to lint only PR titles instead of range-based commit messages, with the git fetch-depth configuration removed. A blank line was added to the README's macOS checklist for formatting.

Changes

Cohort / File(s)	Summary
GitHub Workflow Simplification .github/workflows/commit-lint.yaml	Removed commit message linting between base and head SHAs; now lints only PR title. Removed fetch-depth: 0 git configuration.
Documentation Formatting README.md	Added blank line before "missing developer tools…" bullet in macOS first-run checklist section.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 The workflow now runs lean and light,
PR titles alone shine bright,
A blank line hops into place—
Simple beauty, perfect grace!
✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: removing per-commit linting from the CI workflow and keeping only the PR title check, which aligns with the repo's squash-merge-only policy.
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 cv/commitlint-pr-title-only

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