Document package distribution readiness

[chubes4]

Summary

• Documents the synchronized versioning and release policy for the CLI, core, Playground, and WordPress plugin artifacts.
• Adds install guidance for self-hosted WordPress, Studio/local development, and hosted control-plane environments.
• Clarifies plugin-side CLI binary discovery and expands the release checklist for issue Package and distribute Sandbox Runtime CLI and WordPress plugin #34.

Tests

• npm run build && npm run package-distribution-smoke
• npm run wordpress-plugin-smoke

Issue

• Closes Package and distribute Sandbox Runtime CLI and WordPress plugin #34

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Audited issue Package and distribute Sandbox Runtime CLI and WordPress plugin #34 acceptance against main, drafted the targeted distribution documentation update, ran package/plugin smoke checks, and prepared this PR for Chris to review.