feat: parse categorized changeset summaries

[GuillaumeRx]

Explanation

This is PR 2 of the stacked @metamask/changeset-utils implementation. It adds the canonical parser for categorized Changeset summaries.

parseChangesetSummary accepts top-level bullets with the ADDED, CHANGED, REMOVED, or FIXED prefixes. It supports optional **BREAKING:** markers and keeps nested bullets in the parent description. Invalid summaries throw GrammarError with the source line, stable rule identifier, and a correction hint.

The parser does not parse or validate PR links. Later tooling will generate those links when rendering changelogs.

References

• Related to the approved design: docs/superpowers/specs/2026-10-09-changeset-utils-design.md
• Stacked on the merged package scaffold from PR feat: scaffold changeset utils package #37.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them