feat: auto-detect provider when switching models via /model (#1506)

When typing /model deepseek-chat while on a different provider, the
model name now auto-resolves to the correct provider instead of
silently staying on the wrong one and causing API errors.

Detection priority:
1. Direct provider with credentials (e.g. DEEPSEEK_API_KEY set)
2. OpenRouter catalog match with proper slug remapping
3. Direct provider without creds (clear error beats silent failure)

Also adds DeepSeek as a first-class API-key provider — just set
DEEPSEEK_API_KEY and /model deepseek-chat routes directly.

Bare model names get remapped to proper OpenRouter slugs:
  /model gpt-5.4 → openai/gpt-5.4
  /model claude-opus-4.6 → anthropic/claude-opus-4.6

Salvages the concept from PR #1177 by @virtaava with credential
awareness and OpenRouter slug mapping added.

Co-authored-by: virtaava <virtaava@users.noreply.github.com>

cli.py

@@ -2913,6 +2913,12 @@ class HermesCLI:
                 # Parse provider:model syntax (e.g. "openrouter:anthropic/claude-sonnet-4.5")
                 current_provider = self.provider or self.requested_provider or "openrouter"
                 target_provider, new_model = parse_model_input(raw_input, current_provider)
+                # Auto-detect provider when no explicit provider:model syntax was used
+                if target_provider == current_provider:
+                    from hermes_cli.models import detect_provider_for_model
+                    detected = detect_provider_for_model(new_model, current_provider)
+                    if detected:
+                        target_provider, new_model = detected
                 provider_changed = target_provider != current_provider
 
                 # If provider is changing, re-resolve credentials for the new provider