fix: Use versioned schema URLs in Turborepo skill files (#11994)
## Summary
- Updates `$schema` references in `skills/turborepo/` from the legacy
`https://turborepo.dev/schema.v2.json` to the versioned subdomain format
(`https://v2-8-11.turborepo.dev/schema.json`)
- Adds a step in the `stage-release` Makefile target to automatically
update these URLs on every version bump, so they never go stale again
## Context
The versioned schema URL format
(`https://v{X}-{Y}-{Z}.turborepo.dev/schema.json`) was introduced in
2.7.5 and is what the codemod (`update-versioned-schema-json`) migrates
users to. The skill files were still using the old format, meaning the
AI skill was teaching an outdated pattern.
The new `stage-release` step walks all `.md` files in
`skills/turborepo/`, matches any legacy or outdated versioned schema
URL, and replaces it with the URL for the current release version. The
replacement is idempotent.