Add major release notes for pull_request_reviewer schema removal

[Copilot]

The daily breaking-change report identified a high-severity schema break: removal of on.pull_request_reviewer without major-version release metadata. This PR adds the required major changeset and explicit migration guidance so the break is documented and versioned correctly.

• Release metadata

  • Added .changeset/major-remove-pull-request-reviewer.md with a major bump for gh-aw.
  • Captures the breaking schema change and states that no legacy compatibility is provided.

• Migration guidance

  • Documented the supported replacement path from pull_request_reviewer to pull_request + slash_command.

• Changelog update

  • Added a new entry under Breaking Changes in CHANGELOG.md describing impact, migration path, and semantic-versioning implications.

# Before
on:
  pull_request_reviewer: review

# After
on:
  pull_request:
    - ready_for_review
  slash_command: review

[pelikhan]

@copilot this is a minor bump since it was a experimental feature