ci: allow design type in PR title validation

[opieter-aws]

Description

Adds design to the list of allowed types in the PR Title conventional-commits
check (.github/workflows/pr-title.yml, the amannn/action-semantic-pull-request
step). Design-document PRs use a design: prefix (matching the designs/ folder
convention), but the title-lint only permitted the standard Angular types
(feat, fix, docs, style, refactor, perf, test, build, ci,
chore, revert), so those PRs failed the check with:

Unknown release type "design" found in pull request title

One line added; no other behavior changes.

Related Issues

Unblocks the title check for design-doc PRs such as #2393.

Documentation PR

N/A

Type of Change

Other (please describe): CI / repository tooling change

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

This change only edits a GitHub Actions workflow that validates PR titles.

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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