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.

tests/hermes_cli/test_model_validation.py

@@ -92,6 +92,31 @@ class TestParseModelInput:
         assert provider == "openrouter"
         assert model == "http://localhost:8080/model"
 
+    def test_custom_colon_model_single(self):
+        """custom:model-name → anonymous custom provider."""
+        provider, model = parse_model_input("custom:qwen-2.5", "openrouter")
+        assert provider == "custom"
+        assert model == "qwen-2.5"
+
+    def test_custom_triple_syntax(self):
+        """custom:name:model → named custom provider."""
+        provider, model = parse_model_input("custom:local-server:qwen-2.5", "openrouter")
+        assert provider == "custom:local-server"
+        assert model == "qwen-2.5"
+
+    def test_custom_triple_spaces(self):
+        """Triple syntax should handle whitespace."""
+        provider, model = parse_model_input("custom: my-server : my-model ", "openrouter")
+        assert provider == "custom:my-server"
+        assert model == "my-model"
+
+    def test_custom_triple_empty_model_falls_back(self):
+        """custom:name: with no model → treated as custom:name (bare)."""
+        provider, model = parse_model_input("custom:name:", "openrouter")
+        # Empty model after second colon → no triple match, falls through
+        assert provider == "custom"
+        assert model == "name:"
+
 
 # -- curated_models_for_provider ---------------------------------------------