feat: add auth0 package export core

[zdburrage]

Summary

Adds Auth0 package-mode export on top of the migration package contract.

This includes:

• export-auth0 --package --output-dir
• Package output for users, organizations, memberships, manifest, warnings, and skipped users
• Backward-compatible legacy export-auth0 --output CSV behavior
• Federated/JIT-only Auth0 users skipped by default in package mode
• --include-federated-users override
• Blocked Auth0 users exported as metadata-only rows without credentials
• README usage for Auth0 package mode
• Regression coverage for legacy CSV export and metadata-based package export

Impact

Existing Auth0 CSV export remains available. Package mode adds a richer provider-neutral output directory for future import-package orchestration and SSO/role follow-up phases.

Stack

This is PR 3 of 3. Merge order:

1. feat/package-contract
2. feat/sso-handoff-shared
3. feat/auth0-package-core

Base branch: feat/sso-handoff-shared

Validation

• npm run typecheck
• npm run lint
• npm run format:check
• npm test
• npm run build

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 5 additional findings.