docs: expand architecture boundary guide

[chubes4]

Summary

• Expands the architecture guide into a maintainer map for package responsibilities across runtime-core, runtime-playground, cli, and the optional WordPress plugin adapter.
• Documents the runtime, recipe, artifact, and command/policy lifecycles with links to the modules that define the contracts.
• Reinforces WP Codebox as a generic runtime/artifact substrate, with named products framed only as external consumers.

Closes #346.

Verification

• git diff --check
• npm run build
• npm run check reached recipe-runtime-evidence-smoke after passing the preceding build/smoke steps, then was manually aborted by the user.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the documentation expansion, linked the relevant modules, ran verification commands, and prepared this PR. Chris provided the issue direction and generic-substrate architecture constraint.