build: turbo migration

[soberm]

Description of changes

This PR migrates the monorepo build system from Lerna to Turborepo for improved build performance and caching capabilities.

Key Changes:

• Replaced Lerna with Turborepo as the primary task orchestration tool
• Added turbo.json configuration defining build pipelines with proper task dependencies and caching strategies
• Updated all package.json scripts to use turbo run instead of lerna run
• Removed lerna.json and related Lerna dependencies
• Updated GitHub Actions workflows to use native Yarn workspace commands instead of Lerna
• Modified CONTRIBUTING.md to reflect the new Turborepo-based workflow
• Configured task-level caching for build, test, lint, and ts-coverage tasks
• Set up proper task dependencies (e.g., test depends on ^build)

Benefits:

• Faster builds through intelligent caching and parallelization
• Better task dependency management with explicit dependsOn declarations
• Reduced complexity by leveraging Yarn workspaces directly

Migration Notes:

• yarn bootstrap is no longer needed - just yarn is sufficient
• Watch mode now uses turbo run build:watch instead of Lerna's watch
• Package linking uses yarn workspace <package> link instead of lerna exec

Issue #, if available

N/A - Internal tooling improvement

Description of how you validated changes

• Run Unit and E2E tests
• https://github.com/aws-amplify/amplify-js/actions/runs/21523163106

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.