feat: sync PROVIDER_NAMES with config schema and update README docs

Extract MODEL_PROVIDERS const from config schema as single source of truth
for provider names. PROVIDER_NAMES in TUI commands now imports from schema
instead of maintaining a hardcoded list. Adds tests verifying sync.

Updates README TUI Commands section with /model hot-swap documentation,
supported providers, and runtime model switching examples.

src/frontends/tui/commands.ts

@@ -139,6 +139,8 @@ Commands:
 `.trim();
 }
 
+import { MODEL_PROVIDERS } from '../../config/index.js';
+
 export type ModelAlias = 'local' | 'default' | 'fast' | 'complex' | 'opus' | 'sonnet' | 'haiku' | 'ollama';
 
 // List of all slash commands for autocompletion
@@ -186,8 +188,8 @@ export const COMMAND_TOOLTIPS: Record<string, string> = {
 // Model aliases for /model command autocompletion
 export const MODEL_ALIASES = ['local', 'default', 'fast', 'complex', 'opus', 'sonnet', 'haiku', 'ollama'];
 
-// Provider names for /model <tier> <provider/model> syntax
-export const PROVIDER_NAMES = ['anthropic', 'openai', 'github-copilot', 'gemini', 'bedrock', 'ollama', 'llamacpp'];
+// Provider names for /model <tier> <provider/model> syntax — derived from config schema
+export const PROVIDER_NAMES: readonly string[] = MODEL_PROVIDERS;
 
 // Model alias descriptions
 export const MODEL_TOOLTIPS: Record<string, string> = {