feat(cli): add changeset check command to validate changesets before release - #2293

Summary

Adds a new changeset check command that validates every changeset in .changeset/ is well-formed and only references packages that still exist in the workspace. Designed to run in CI on main (or on PRs) to catch malformed or stale changesets before they break changeset version during a release.

Problem

When changesets are created or edited by hand (or by an AI tool), they can end up:

• malformed — missing/invalid frontmatter, bad YAML, invalid version types, etc.
• stale — referencing packages that were since deleted from the workspace

These errors currently surface during changeset version on the release branch, blocking the release. changeset status catches them too, but it also runs git (diffing against baseBranch/--since) and exits 1 when there are changed packages without changesets — which makes it unsuitable as a pure validation check on main.

Solution

changeset check reuses the existing readChangesets + assembleReleasePlan pipeline:

• readChangesets parses each .changeset/*.md file → throws on malformed format (missing frontmatter, invalid YAML, bad version type, etc.)
• assembleReleasePlan validates that every referenced package exists in the workspace → throws Found changeset X for package Y which is not in the workspace

It reads only the working tree (no --since, no git diff), so it works in shallow CI clones and merge-commit checkouts without depending on git history.

# .github/workflows/release.yml
- run: changeset check
- run: changeset version

What it does NOT do

• Does not use git (unlike status, which calls git diff against baseBranch/--since)
• Does not fail when there are changed packages without changesets (that is status's job, and intentionally not recommended for CI)
• Does not check only changesets added in the current PR — it validates all changesets present on disk in the current checkout
• Stops at the first invalid changeset (error message names the file + reason — good enough; a collect-all-errors mode can be added later if needed)

Files

• packages/cli/src/commands/check/index.ts — the command (~45 lines, reuses readChangesets + assembleReleasePlan)
• packages/cli/src/cli.ts — registers the check command
• packages/cli/src/commands/check/__tests__/check.test.ts — 7 tests (valid, empty --empty changeset, no changesets, stale package ref, malformed frontmatter, invalid version type, no-fail-when-changed-packages-have-no-changeset)
• packages/cli/src/cli.test.ts — CLI wiring test
• docs/command-line-options.md + docs/checking-for-changesets.md — docs

Test plan

• pnpm types:check
• pnpm lint
• pnpm format
• pnpm test run packages/cli/src/cli.test.ts (30 passed)
• pnpm test run packages/cli/src/commands/check (7 passed)
• Manual: node packages/cli/bin.js check on this repo → All 2 changeset(s) are valid.
• Manual: malformed changeset → exits 1 with file + reason
• Manual: stale package reference → exits 1 with Found changeset X for package Y which is not in the workspace