fix: respect CLAUDE_CONFIG_DIR for plugin paths (#626)

Add MARKETPLACE_ROOT constant to paths.ts and update 5 source files to use
centralized path constants instead of hardcoded ~/.claude paths. Preserves
backwards compatibility when CLAUDE_CONFIG_DIR is not set.

Based on PR #634 by @Kuroakira, cherry-picked onto main due to build artifact
merge conflicts (source changes applied cleanly).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/infrastructure/HealthMonitor.ts

@@ -10,9 +10,9 @@
  */
 
 import path from 'path';
-import { homedir } from 'os';
 import { readFileSync } from 'fs';
 import { logger } from '../../utils/logger.js';
+import { MARKETPLACE_ROOT } from '../../shared/paths.js';
 
 /**
  * Check if a port is in use by querying the health endpoint
@@ -100,8 +100,7 @@ export async function httpShutdown(port: number): Promise<boolean> {
  * This is the "expected" version that should be running
  */
 export function getInstalledPluginVersion(): string {
-  const marketplaceRoot = path.join(homedir(), '.claude', 'plugins', 'marketplaces', 'thedotmack');
-  const packageJsonPath = path.join(marketplaceRoot, 'package.json');
+  const packageJsonPath = path.join(MARKETPLACE_ROOT, 'package.json');
   const packageJson = JSON.parse(readFileSync(packageJsonPath, 'utf-8'));
   return packageJson.version;
 }