Advance publication contract fixes

[chubes4]

Summary

• pin the developer documentation workflow to Docs Agent 972ceb8a9ebcfafa1f88edc4fba173065415698a
• advance the immutable WP Codebox producer checkout and expected workflow release to v0.12.28
• verify the corrected nested publication projection and case-insensitive canonical repository identity end to end

How to test

1. Check out Docs Agent at 972ceb8a9ebcfafa1f88edc4fba173065415698a.
2. Check out WP Codebox at 6457dcae4cb8cf8e76cd174377de8cbee3ee12cb.
3. Run pnpm install --frozen-lockfile.
4. Run pnpm build.
5. Run pnpm verify.
6. Run DOCS_AGENT_DIR=/path/to/docs-agent WP_CODEBOX_DIR=/path/to/wp-codebox pnpm test.
7. Run actionlint .github/workflows/*.yml.
8. Run git diff --check origin/trunk...HEAD.

All commands pass locally against the exact producer commits.

Compatibility

No extension paths, public interfaces, input schemas, or package revisions change. This advances the nested runner from WP Codebox v0.12.27 to v0.12.28 and consumes the corrected Docs Agent publication projection.

Source relationships

• Project the nested runner workspace pull-request URL docs-agent#162
• Project nested publication pull request URL docs-agent#163
• Advance Docs Agent to WP Codebox v0.12.28 docs-agent#164
• Publication verification rejects canonical repository URL casing wp-codebox#1874
• Accept canonical GitHub repository casing wp-codebox#1875
• Correct developer workflow provenance documentation #121

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with OpenAI GPT-5.6 Sol
• Used for: Updated immutable producer pins, strengthened the cross-repository contract, and ran the complete repository-native validation chain. Chris reviewed and owns the change.