fix: remove dead LLM_MODEL env var — add migration to clear stale .env entries (#6543)

The old setup wizard (pre-March 2026) wrote LLM_MODEL to ~/.hermes/.env
across 12 provider flows. Commit 9302690e removed the writes but never
cleaned up existing .env files, leaving a dead variable that:
- Nothing in the codebase reads (zero os.getenv calls)
- The docs incorrectly claimed the gateway still used as fallback
- Caused user confusion when debugging model resolution issues

Changes:
- config.py: Bump _config_version 12 → 13, add migration to clear
  LLM_MODEL and OPENAI_MODEL from .env (both dead since March 2026)
- environment-variables.md: Remove LLM_MODEL row, fix HERMES_MODEL
  description to stop referencing it
- providers.md: Update deprecation notice from 'deprecated' to 'removed'

website/docs/integrations/providers.md

@@ -230,7 +230,7 @@ model:
 ```
 
 :::warning Legacy env vars
-`OPENAI_BASE_URL` and `LLM_MODEL` in `.env` are **deprecated**. `OPENAI_BASE_URL` is no longer consulted for endpoint resolution — `config.yaml` is the single source of truth. The CLI ignores `LLM_MODEL` entirely (only the gateway reads it as a fallback). Use `hermes model` or edit `config.yaml` directly — both persist correctly across restarts and Docker containers.
+`OPENAI_BASE_URL` and `LLM_MODEL` in `.env` are **removed**. Neither is read by any part of Hermes — `config.yaml` is the single source of truth for model and endpoint configuration. If you have stale entries in your `.env`, they are automatically cleared on the next `hermes setup` or config migration. Use `hermes model` or edit `config.yaml` directly.
 :::
 
 Both approaches persist to `config.yaml`, which is the source of truth for model, provider, and base URL.