Add InvalidFrontmatter to ERROR_CODE_MESSAGES and guard against future gaps

ErrorCode.InvalidFrontmatter was introduced in d97f43b but its corresponding
entry in ERROR_CODE_MESSAGES was omitted, causing a TypeScript compilation
error in the 4.1.5 release.

Two guards added to prevent recurrence:
- src/constants.test.ts: asserts that every ErrorCode value has a message
  entry, so omitting one fails the test suite before a release can proceed.
- .github/workflows/release.yml: runs typecheck and the full test suite
  before attesting and uploading artifacts, so a type error in the release
  build is caught by CI rather than reaching users.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>