refactor: extract CLI recipe source helpers

[chubes4]

Summary

• Extract recipe source/preparation responsibilities from packages/cli/src/index.ts into packages/cli/src/recipe-sources.ts.
• Keep CLI behavior unchanged while leaving validation, dry-run planning, routing, and runtime execution in index.ts.
• Preserve recipe source provenance, workspace prep/cleanup, extra plugin prep, staged file prep, download guards, scaffolds, and plugin activation helpers behind focused exports.

Verification

• npm run build
• npm run recipe-staged-files-smoke
• npm run recipe-workspace-seed-excludes-smoke
• npm run recipe-heavyweight-plugin-runtime-smoke (first attempt hit local timeout with no failure output; retry passed)
• npm run recipe-dry-run-smoke

Tracker

• Closes Architecture cleanup: split CLI entrypoint into command and recipe subsystems #342

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the focused helper extraction, ran verification, and prepared this PR for Chris to review.