Refactor runtime-core recipe schema contracts

[chubes4]

Summary

• Extract the workspace recipe JSON schema factory and related schema option/types from packages/runtime-core/src/index.ts into packages/runtime-core/src/recipe-schema.ts.
• Re-export the new module from runtime-core so existing public imports from @chubes4/wp-codebox-core continue to work unchanged.
• Keep the extraction behavior-preserving with no schema content changes.

Tests

• npm install (ran because the isolated worktree had no node_modules; reverted incidental package-lock.json version churn)
• npm run build
• npm run package-distribution-smoke
• npm run discovery-command-smoke
• npm run recipe-dry-run-smoke
• npm run recipe-workflow-phases-smoke
• npm run recipe-site-seed-smoke
• npm run recipe-staged-files-smoke
• npm run recipe-workspace-seed-excludes-smoke timed out after 20 minutes; its lingering recipe-run child processes were stopped
• npm run recipe-heavyweight-plugin-runtime-smoke timed out after 10 minutes; its lingering recipe-run child process was stopped

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted and verified this focused runtime-core recipe schema extraction; Chris remains responsible for review and merge.