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/reference/environment-variables.md

@@ -53,8 +53,7 @@ All variables go in `~/.hermes/.env`. You can also set them with `hermes config
 | `OPENCODE_GO_API_KEY` | OpenCode Go API key — $10/month subscription for open models ([opencode.ai](https://opencode.ai/auth)) |
 | `OPENCODE_GO_BASE_URL` | Override OpenCode Go base URL |
 | `CLAUDE_CODE_OAUTH_TOKEN` | Explicit Claude Code token override if you export one manually |
-| `HERMES_MODEL` | Preferred model name (checked before `LLM_MODEL`, used by gateway) |
-| `LLM_MODEL` | Default model name (fallback when not set in config.yaml) |
+| `HERMES_MODEL` | Override model name at process level (used by cron scheduler; prefer `config.yaml` for normal use) |
 | `VOICE_TOOLS_OPENAI_KEY` | Preferred OpenAI key for OpenAI speech-to-text and text-to-speech providers |
 | `HERMES_LOCAL_STT_COMMAND` | Optional local speech-to-text command template. Supports `{input_path}`, `{output_dir}`, `{language}`, and `{model}` placeholders |
 | `HERMES_LOCAL_STT_LANGUAGE` | Default language passed to `HERMES_LOCAL_STT_COMMAND` or auto-detected local `whisper` CLI fallback (default: `en`) |