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.

hermes_cli/commands.py

@@ -18,6 +18,7 @@ COMMANDS = {
     "/tools": "List available tools",
     "/toolsets": "List available toolsets",
     "/model": "Show or change the current model",
+    "/provider": "Show available providers and current provider",
     "/prompt": "View/set custom system prompt",
     "/personality": "Set a predefined personality",
     "/clear": "Clear screen and reset conversation (fresh start)",