Summary

• Invalidate the cached EnvConfig after _ensure_dotenv() loads a real .env file.
• Ensure the CLI welcome panel, /settings, and dotenv diagnostic use the configured provider and model.
• Add a regression reproducing configuration cached before dotenv loading.

Root cause

Early CLI imports could initialize the EnvConfig singleton before .env was loaded. _ensure_dotenv() populated os.environ but did not invalidate that stale singleton.

Validation

• 6 dotenv tests passed
• 131 focused config/provider/preflight/LLM tests passed
• 5,384 broader tests passed, with 6 evidence-backed baseline/environment exclusions
• Compile, Ruff, env-var gate, and diff checks passed

Closes #541