Add reusable Docs Agent workflow

[chubes4]

Summary

• Add a manual Docs Agent workflow that consumes the reusable Extra-Chill/docs-agent Data Machine bundle.
• Add a minimal Playground driver plugin and runner script for importing and running the external bundle against Automattic/agents-api.
• Force writable docs paths to README.md and docs/** through the runner config so generated file writes stay scoped.

Testing

• php -l tests/playground-ci/component/agents-api-docs-agent-driver.php && bash -n tests/playground-ci/scripts/run-docs-agent.sh && git diff --check
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/docs-agent.yml"); puts "workflow yaml ok"'
• homeboy lint --path . --extension wordpress --changed-since origin/main

Dependencies

• Add reusable Docs Agent bundle Extra-Chill/docs-agent#1 provides the reusable bundle.
• Force Data Machine agent tool parameters Extra-Chill/homeboy-extensions#490 provides forced_parameters support for hard path-scope enforcement.
• Live workflow proof should run after both dependency PRs are merged, or by overriding docs_agent_ref and homeboy_extensions_ref in workflow_dispatch.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the workflow, runner script, and validation path for consuming the reusable Docs Agent bundle; Chris remains responsible for review and merge.