Use Docs Agent reusable maintain workflow

[chubes4]

Summary

• Replace the legacy Docs Agent runner wiring with Automattic/docs-agent/.github/workflows/maintain-docs.yml@main.
• Preserve product-level controls for audience, bootstrap vs maintenance runs, model, Docs Agent ref, base ref, canonical docs branch, writable paths, and prompt.
• Pass Agents API verification through the reusable workflow with php tests/no-product-imports-smoke.php.

Validation

• php tests/no-product-imports-smoke.php
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/docs-agent.yml"); puts "YAML OK"'
• git diff --check

Dependency

• Requires the reusable maintain-docs.yml workflow on Automattic/docs-agent@main.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode
• Model: openai/gpt-5.5
• Used for: Updated the Docs Agent workflow migration and drafted the PR description; Chris remains responsible for review and merge.