Fix Homeboy project detection when site URLs are blank

[chubes4]

Summary

• Scope Studio WP-CLI site URL detection to the target WordPress path with studio wp option get siteurl --path=<site> so upgrade/setup resolves the intended site instead of the ambient/default Studio context.
• Keep a basename fallback only as a last-resort guard when WP-CLI returns no site URL.
• Harden Homeboy project resolution so absolute site paths do not slugify to an empty project id, and preserve empty homeboy.json as a skip condition.

Testing

• bash tests/*.sh
• node tests/effective-prompt/run.mjs
• Canary: ./upgrade.sh --skip-plugins --wp-path /Users/chubes/Studio/intelligence-chubes4 from this branch installed the generated AGENTS.md guidance without the empty Homeboy project warning.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosed the failed canary path, drafted the path-scoped resolver fix and regression test, ran local verification, and prepared this PR for Chris to review.