Remove legacy Data Machine Docs Agent bundles

[chubes4]

Summary

• Remove the obsolete Data Machine manifests, flows, pipelines, memory envelopes, and their legacy-only validation.
• Keep the five Agents API-native .agent.json packages as Docs Agent's sole executable instruction authority.
• Keep maintain-docs.yml on immutable native package selection and WP Codebox's generic runner/publication boundary.
• Replace legacy bundle validation with focused native package and workflow-lane validation.

Fixes #124.

Architecture

Docs Agent no longer carries two execution formats. The canonical lanes are:

• technical bootstrap
• technical maintenance
• user bootstrap
• user maintenance
• skills maintenance

Each lane maps directly to one immutable native package. Docs Agent owns package selection and workspace-editing behavior; WP Codebox owns execution, verification, commit, push, and pull-request publication.

Compatibility

This intentionally removes the legacy Data Machine extension paths under bundles/*/{manifest.json,flows,pipelines,memory}. Direct consumers of those files must migrate to the corresponding bundles/*/native/*.agent.json package.

Consumers calling .github/workflows/maintain-docs.yml already use native packages and require no migration. Product-level workflow inputs remain unchanged.

How to test

1. Check out this PR branch.
2. Run php tests/validate-docs-agent-packages.php.
3. Run php tests/validate-external-native-package-sources.php.
4. Run php tests/repair-docs-links-smoke.php.
5. Clone Automattic/wp-codebox at tag v0.12.14 into a sibling wp-codebox directory, then run WP_CODEBOX_DIR=../wp-codebox php tests/validate-wp-codebox-run-agent-task-contract.php.
6. Clone Automattic/agents-api into a sibling agents-api directory, then run AGENTS_API_DIR=../agents-api php tests/native-agent-import.php.
7. Confirm all five package lanes import and all 36 native package assertions pass.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode and Homeboy
• Model: OpenAI GPT-5.6 Terra and GPT-5.6 Sol
• Used for: Removing the obsolete Data Machine-shaped bundle surface, updating native package documentation and tests, and verifying the canonical Docs Agent workflow with Chris.