feat: /provider command + fix gateway bugs + harden parse_model_input

/provider command (CLI + gateway):
  Shows all providers with auth status (✓/✗), aliases, and active marker.
  Users can now discover what provider names work with provider:model syntax.

Gateway bugs fixed:
  - Config was saved even when validation.persist=False (told user 'session
    only' but actually persisted the unvalidated model)
  - HERMES_INFERENCE_PROVIDER env var not set on provider switch, causing
    the switch to be silently overridden if that env var was already set

parse_model_input hardened:
  - Colon only treated as provider delimiter if left side is a recognized
    provider name or alias. 'anthropic/claude-3.5-sonnet:beta' now passes
    through as a model name instead of trying provider='anthropic/claude-3.5-sonnet'.
  - HTTP URLs, random colons no longer misinterpreted.

56 tests passing across model validation, CLI commands, and integration.

tests/hermes_cli/test_model_validation.py

@@ -67,6 +67,17 @@ class TestParseModelInput:
         assert provider == "openrouter"
         assert model == ":something"
 
+    def test_unknown_prefix_colon_not_treated_as_provider(self):
+        """Colons are only provider delimiters if the left side is a known provider."""
+        provider, model = parse_model_input("anthropic/claude-3.5-sonnet:beta", "openrouter")
+        assert provider == "openrouter"
+        assert model == "anthropic/claude-3.5-sonnet:beta"
+
+    def test_http_url_not_treated_as_provider(self):
+        provider, model = parse_model_input("http://localhost:8080/model", "openrouter")
+        assert provider == "openrouter"
+        assert model == "http://localhost:8080/model"
+
 
 # -- curated_models_for_provider ---------------------------------------------