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/config/index.ts

@@ -1,2 +1,2 @@
 export { loadConfig, deepMerge } from './loader.js';
-export { configSchema, type Config, type TelegramConfig, type ModelConfig, type CronJobConfig, type AgentsConfig, type CompactionConfig, type ToolProfile, type ToolOverrideConfig, type ToolsConfig, type SandboxConfig, type AgentConfigEntry, type RoutingConfig, type ServerConfig } from './schema.js';
+export { configSchema, MODEL_PROVIDERS, type ModelProvider, type Config, type TelegramConfig, type ModelConfig, type CronJobConfig, type AgentsConfig, type CompactionConfig, type ToolProfile, type ToolOverrideConfig, type ToolsConfig, type SandboxConfig, type AgentConfigEntry, type RoutingConfig, type ServerConfig } from './schema.js';

src/config/schema.ts

@@ -38,8 +38,13 @@ const serverSchema = z.object({
   lock: z.boolean().default(false),
 });
 
+/** All supported model provider identifiers. Used by the config schema and TUI autocompletion. */
+export const MODEL_PROVIDERS = ['anthropic', 'openai', 'gemini', 'ollama', 'llamacpp', 'openrouter', 'bedrock', 'github', 'zhipuai', 'xai'] as const;
+
+export type ModelProvider = (typeof MODEL_PROVIDERS)[number];
+
 const modelConfigBaseSchema = z.object({
-  provider: z.enum(['anthropic', 'openai', 'gemini', 'ollama', 'llamacpp', 'openrouter', 'bedrock', 'github', 'zhipuai', 'xai']),
+  provider: z.enum(MODEL_PROVIDERS),
   model: z.string(),
   endpoint: z.string().optional(),
   api_key: z.string().optional(),