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'

hermes_cli/config.py

@@ -569,7 +569,7 @@ DEFAULT_CONFIG = {
     },
 
     # Config schema version - bump this when adding new required fields
-    "_config_version": 12,
+    "_config_version": 13,
 }
 
 # =============================================================================
@@ -1701,6 +1701,21 @@ def migrate_config(interactive: bool = True, quiet: bool = False) -> Dict[str, A
                         ep = providers_dict[key]
                         print(f"    → {key}: {ep.get('api', '')}")
 
+    # ── Version 12 → 13: clear dead LLM_MODEL / OPENAI_MODEL from .env ──
+    # These env vars were written by the old setup wizard but nothing reads
+    # them anymore (config.yaml is the sole source of truth since March 2026).
+    # Stale entries cause user confusion — see issue report.
+    if current_ver < 13:
+        for dead_var in ("LLM_MODEL", "OPENAI_MODEL"):
+            try:
+                old_val = get_env_value(dead_var)
+                if old_val:
+                    save_env_value(dead_var, "")
+                    if not quiet:
+                        print(f"  ✓ Cleared {dead_var} from .env (no longer used — config.yaml is source of truth)")
+            except Exception:
+                pass
+
     if current_ver < latest_ver and not quiet:
         print(f"Config version: {current_ver} → {latest_ver}")