Add command to review breaking changes. /review-breaking-changes in a plan or spec

[jabrena]

User Story

As a repository maintainer or Java rule author
I want to use a /review-breaking-changes command from a plan or spec
So that I can identify potential breaking changes before implementation or release work proceeds.

Acceptance Criteria

Feature: Breaking Change Review Command

Background:
  Given an OpenSpec plan or spec exists for a proposed repository change

@acceptance-test
Scenario: Review a plan or spec for breaking-change risk
  Given a maintainer has a plan or spec describing a proposed change
  When the maintainer runs `/review-breaking-changes` against that plan or spec
  Then the command reports potential breaking changes
  And the report highlights affected contracts, generated outputs, commands, skills, or documentation where applicable
  And the report provides actionable guidance for follow-up validation

INVEST Validation

• Independent: The command can be added as a standalone project command without depending on another unfinished command.
• Negotiable: The exact report sections and review depth can be refined while preserving the core goal of breaking-change detection.
• Valuable: Maintainers get earlier visibility into compatibility risks before implementation or release promotion.
• Estimable: The scope is bounded to adding one command that reviews an existing plan or spec and emits a structured risk report.
• Small: The first iteration can focus on command documentation and expected review output for plan/spec inputs.
• Testable: Completion can be verified by running the command against a sample plan or spec and checking that breaking-change risks are reported.

[jabrena]

#927