Prefer Carbon dashboard env vars

[w287346141]

Summary

• Prefer CARBONCODE_DASHBOARD_HOST and CARBONCODE_DASHBOARD_TOKEN for dashboard configuration.
• Keep REASONIX_DASHBOARD_HOST and REASONIX_DASHBOARD_TOKEN as legacy fallbacks.
• Move dashboard host/token resolution into a small helper module so the env priority can be tested without importing the CLI entrypoint.
• Add tests for flag precedence, Carbon env priority, legacy fallback behavior, and short-token rejection.

Validation

• npm.cmd test -- --run tests/dashboard-options.test.ts
• npm.cmd run typecheck
• npm.cmd run lint