feat: /model command — models.dev primary database + --provider flag (#5181)

Full overhaul of the model/provider system.

## What changed
- models.dev (109 providers, 4000+ models) as primary database for provider identity AND model metadata
- --provider flag replaces colon syntax for explicit provider switching
- Full ModelInfo/ProviderInfo dataclasses with context, cost, capabilities, modalities
- HermesOverlay system merges models.dev + Hermes-specific transport/auth/aggregator flags
- User-defined endpoints via config.yaml providers: section
- /model (no args) lists authenticated providers with curated model catalog
- Rich metadata display: context window, max output, cost/M tokens, capabilities
- Config migration: custom_providers list → providers dict (v11→v12)
- AIAgent.switch_model() for in-place model swap preserving conversation

## Files
agent/models_dev.py, hermes_cli/providers.py, hermes_cli/model_switch.py,
hermes_cli/model_normalize.py, cli.py, gateway/run.py, run_agent.py,
hermes_cli/config.py, hermes_cli/commands.py

hermes_cli/commands.py

@@ -84,6 +84,7 @@ COMMAND_REGISTRY: list[CommandDef] = [
     # Configuration
     CommandDef("config", "Show current configuration", "Configuration",
                cli_only=True),
+    CommandDef("model", "Switch model for this session", "Configuration", args_hint="[model] [--global]"),
     CommandDef("provider", "Show available providers and current provider",
                "Configuration"),
     CommandDef("prompt", "View/set custom system prompt", "Configuration",