Fix skill discoverability and Cursor marketplace parity

[arijitroy003]

Summary

• tile.json / README: Add using-dbt-state which exists on disk but was not registered in tile.json or listed in the README skill table
• Cursor marketplace.json: Add missing dbt-migration and dbt-extras plugin entries to .cursor-plugin/marketplace.json, matching the existing .claude-plugin/marketplace.json
• Cursor plugin.json: Add .cursor-plugin/plugin.json for dbt-migration and dbt-extras plugins (these existed for the dbt plugin but were missing for the other two)

Context

Addresses #99. The validate_repo.py script catches the tile.json gap, and the Cursor marketplace inconsistency was reported in the same issue.

Test plan

• uv run python scripts/validate_repo.py passes with no failures
• using-dbt-state is discoverable via tile.json
• Cursor marketplace lists all three plugins (dbt, dbt-migration, dbt-extras)

[b-per]

Thanks! We will likely incorporate adding using-dbt-state to tile.json but for Cursor the team mentioned to us a few months ago that they supported only one plugin, hence why we only picked the "main" dbt one.

[arijitroy003]

Thanks for the context @b-per — makes sense about the Cursor limitation. Would it be useful to land just the tile.json change for using-dbt-state discoverability and drop the Cursor marketplace changes? Happy to split or adjust the PR however works best.