feat(coding-agent): configurable app name and config dir for forks (#95)

- Add piConfig to package.json for app name and config directory
- Consolidate paths.ts into config.ts with clearer naming
- Fix Bun binary detection (changed from %7EBUN to $bunfs)
- Update all hardcoded paths to use config.ts exports
- getThemesDir() for built-in themes, getCustomThemesDir() for user themes

packages/coding-agent/src/tui/model-selector.ts

@@ -82,7 +82,7 @@ export class ModelSelectorComponent extends Container {
 	}
 
 	private async loadModels(): Promise<void> {
-		// Load available models fresh (includes custom models from ~/.pi/agent/models.json)
+		// Load available models fresh (includes custom models from models.json)
 		const { models: availableModels, error } = await getAvailableModels();
 
 		// If there's an error loading models.json, we'll show it via the "no models" path