feat(ai): add model caching, settings, and commands

- Introduce model caching with persistence to preferences
- Add provider default model and custom settings management
- Implement `/ai switch`, `/ai models`, and improve `/ai start`
- Add model name autocomplete for chat commands
- Update command definitions and help text
- Add unit tests for new functionality

src/command/cmd_defs.c

@@ -2794,8 +2794,9 @@ static const struct cmd_t command_defs[] = {
               { "remove", cmd_ai_remove },
               { "start", cmd_ai_start },
               { "clear", cmd_ai_clear },
-              { "correct", cmd_ai_correct },
-              { "providers", cmd_ai_providers })
+              { "providers", cmd_ai_providers },
+              { "switch", cmd_ai_switch },
+              { "models", cmd_ai_models })
       CMD_MAINFUNC(cmd_ai)
       CMD_TAGS(
               CMD_TAG_CHAT)
@@ -2803,43 +2804,52 @@ static const struct cmd_t command_defs[] = {
               "/ai",
               "/ai set provider <name> <url>",
               "/ai set token <provider> <token>",
-              "/ai set org <provider> <org_id>",
+              "/ai set default-provider <provider>",
+              "/ai set default-model <provider> <model>",
+              "/ai set custom <provider> <setting> <value>",
               "/ai remove provider <name>",
               "/ai providers",
-              "/ai providers-list",
-              "/ai start [<provider>/<model>]",
-              "/ai model <model>",
-              "/ai clear",
-              "/ai correct <message>")
+              "/ai start [<provider>] [<model>]",
+              "/ai switch <provider> [<model>]",
+              "/ai switch <model>",
+              "/ai models <provider>",
+              "/ai clear")
       CMD_DESC(
               "Interact with AI models via OpenAI-compatible APIs. "
               "Supports multiple providers (openai, perplexity, custom). "
-              "Each provider has its own API key and endpoint configuration. "
+              "Each provider has its own API key, endpoint, default model, and settings. "
               "Chat history is maintained per session and not persisted locally.")
       CMD_ARGS(
               { "", "Display current AI settings and configured providers" },
               { "set provider <name> <url>", "Add or update a provider with custom API endpoint" },
               { "set token <provider> <token>", "Set API token for a provider (e.g., openai, perplexity)" },
-              { "set org <provider> <org_id>", "Set organization ID for a provider (optional)" },
+              { "set default-provider <provider>", "Set global default provider for /ai start" },
+              { "set default-model <provider> <model>", "Set default model for a provider" },
+              { "set custom <provider> <setting> <value>", "Set provider-level setting (e.g., tools, search)" },
               { "remove provider <name>", "Remove a custom provider" },
-              { "providers", "List all available providers" },
-              { "providers-list", "List configured providers with keys status" },
-              { "start [<provider>/<model>]", "Start new AI chat (e.g., openai/gpt-4o)" },
-              { "model <model>", "Change model in current chat window" },
-              { "clear", "Clear current chat history" },
-              { "correct <message>", "Correct last user message and get new response" })
+              { "providers", "List configured providers with full details" },
+              { "start [<provider>] [<model>]", "Start new AI chat (space-separated, uses defaults if omitted)" },
+              { "switch <provider> [<model>]", "Switch to different provider (and optionally model)" },
+              { "switch <model>", "Change model in current session (keeps provider)" },
+              { "models <provider>", "Fetch and display available models for provider" },
+              { "clear", "Clear current chat history" })
       CMD_EXAMPLES(
               "/ai",
               "/ai set token openai sk-xxx",
               "/ai set token perplexity pplx-xxx",
               "/ai set provider custom https://my-api.com/v1/chat/completions",
-              "/ai set org openai my-org-id",
+              "/ai set default-provider perplexity",
+              "/ai set default-model perplexity sonar",
+              "/ai set custom perplexity tools enabled",
               "/ai remove provider custom",
-              "/ai start openai/gpt-4o",
-              "/ai start perplexity/sonar",
-              "/ai providers-list",
-              "/ai clear",
-              "/ai correct I meant something else")
+              "/ai start",
+              "/ai start perplexity",
+              "/ai start perplexity sonar",
+              "/ai start openai gpt-4o",
+              "/ai switch gpt-4o",
+              "/ai switch openai gpt-5.4-nano",
+              "/ai models perplexity",
+              "/ai clear")
     },
 
     // NEXT-COMMAND (search helper)