chore: better welcome messgae, throw on config parse error

add instructions for gateway mode, change config order, and don't
hard-code the config path, pull it from the default config

src/config/types.ts

@@ -112,6 +112,7 @@ export type HeartbeatConfig = z.infer<typeof HeartbeatConfigSchema>;
 // ---------------------------------------------------------------------------
 
 export const ConfigSchema = z.object({
+  providers: ProvidersConfigSchema.default(() => ({})),
   agent: AgentConfigSchema.default(() => ({
     model: 'anthropic/claude-sonnet-4-5',
     workspacePath: '~/.config/nanobot',
@@ -120,13 +121,12 @@ export const ConfigSchema = z.object({
     temperature: 0.7,
     maxToolIterations: 40,
   })),
-  providers: ProvidersConfigSchema.default(() => ({})),
+  heartbeat: HeartbeatConfigSchema.default(() => ({ enabled: false, intervalMinutes: 30 })),
   channels: ChannelsConfigSchema.default(() => ({ sendProgress: true, sendToolHints: true })),
   tools: ToolsConfigSchema.default(() => ({
     exec: { timeout: 120, denyPatterns: [], restrictToWorkspace: false },
     web: {},
     restrictToWorkspace: false,
   })),
-  heartbeat: HeartbeatConfigSchema.default(() => ({ enabled: false, intervalMinutes: 30 })),
 });
 export type Config = z.infer<typeof ConfigSchema>;