chore(blueprint): track main branch instead of version tag

[firestar300]

Updates the blueprint configuration to pull the plugin resource from the main branch rather than a specific version tag. This ensures the Playground environment reflects the latest development changes.

---

Note

Medium Risk
Low code impact (config-only), but switching from a pinned tag to main makes Playground builds non-deterministic and can unexpectedly change or break the demo as upstream evolves.

Overview
Updates the WordPress Playground blueprint to install BeAPI/blockparty-modal from the main branch instead of the pinned 1.0.7 tag (refType tag -> branch).

This makes the Playground environment track the latest plugin code rather than a fixed, reproducible version.

^{Reviewed by Cursor Bugbot for commit 9ca9d27. Bugbot is set up for automated code reviews on this repo. Configure here.}