refactor(ai): remove default command and enforce strict window context

- Remove `/ai set default-provider` command and related autocomplete logic
- Remove global `wins_get_ai()` window accessor function
- Enforce strict requirement that `/ai model`, `/ai switch`, `/ai models`, and `/ai clear` commands must be executed from within an active AI chat window
- Update `/ai start` to require `<provider>` argument

BREAKING CHANGE: `/ai model`, `/ai switch`, `/ai models`, and `/ai clear` commands now require an active AI window context. The `/ai set default-provider` command has been removed.

src/command/cmd_defs.c

@@ -2804,7 +2804,6 @@ static const struct cmd_t command_defs[] = {
               "/ai",
               "/ai set provider <name> <url>",
               "/ai set token <provider> <token>",
-              "/ai set default-provider <provider>",
               "/ai set default-model <provider> <model>",
               "/ai set custom <provider> <setting> <value>",
               "/ai remove provider <name>",
@@ -2823,14 +2822,12 @@ static const struct cmd_t command_defs[] = {
               { "", "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 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 configured providers with full details" },
-              { "start [<provider>] [<model>]", "Start new AI chat (space-separated, uses defaults if omitted)" },
+              { "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(
@@ -2838,7 +2835,6 @@ static const struct cmd_t command_defs[] = {
               "/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 default-provider perplexity",
               "/ai set default-model perplexity sonar",
               "/ai set custom perplexity tools enabled",
               "/ai remove provider custom",