feat(model): /model command overhaul — Phases 2, 3, 5

* feat(model): persist base_url on /model switch, auto-detect for bare /model custom

Phase 2+3 of the /model command overhaul:

Phase 2 — Persist base_url on model switch:
- CLI: save model.base_url when switching to a non-OpenRouter endpoint;
  clear it when switching away from custom to prevent stale URLs
  leaking into the new provider's resolution
- Gateway: same logic using direct YAML write

Phase 3 — Better feedback and edge cases:
- Bare '/model custom' now auto-detects the model from the endpoint
  using _auto_detect_local_model() and saves all three config values
  (model, provider, base_url) atomically
- Shows endpoint URL in success messages when switching to/from
  custom providers (both CLI and gateway)
- Clear error messages when no custom endpoint is configured
- Updated test assertions for the additional save_config_value call

Fixes #2562 (Phase 2+3)

* feat(model): support custom:name:model triple syntax for named custom providers

Phase 5 of the /model command overhaul.

Extends parse_model_input() to handle the triple syntax:
  /model custom:local-server:qwen → provider='custom:local-server', model='qwen'
  /model custom:my-model          → provider='custom', model='my-model' (unchanged)

The 'custom:local-server' provider string is already supported by
_get_named_custom_provider() in runtime_provider.py, which matches
it against the custom_providers list in config.yaml. This just wires
the parsing so users can do it from the /model slash command.

Added 4 tests covering single, triple, whitespace, and empty model cases.

hermes_cli/models.py

@@ -345,6 +345,15 @@ def parse_model_input(raw: str, current_provider: str) -> tuple[str, str]:
         provider_part = stripped[:colon].strip().lower()
         model_part = stripped[colon + 1:].strip()
         if provider_part and model_part and provider_part in _KNOWN_PROVIDER_NAMES:
+            # Support custom:name:model triple syntax for named custom
+            # providers.  ``custom:local:qwen`` → ("custom:local", "qwen").
+            # Single colon ``custom:qwen`` → ("custom", "qwen") as before.
+            if provider_part == "custom" and ":" in model_part:
+                second_colon = model_part.find(":")
+                custom_name = model_part[:second_colon].strip()
+                actual_model = model_part[second_colon + 1:].strip()
+                if custom_name and actual_model:
+                    return (f"custom:{custom_name}", actual_model)
             return (normalize_provider(provider_part), model_part)
     return (current_provider, stripped)