fix: Address Issue #23 - Resource management for singleton ParentPomResolver

Added JVM shutdown hook to automatically clean up ParentPomResolver resources:
- Shutdown hook calls close() on the singleton instance when JVM exits
- This ensures repositorySystem.shutdown() is called exactly once
- No need for individual MuleApplication instances to close the shared resolver

Updated AGENTS.md to document the singleton resource management:
- Documented that singleton is automatically cleaned up via shutdown hook
- Clarified that application code should NOT call close() on the singleton
- Explained when to use new ParentPomResolver() for custom lifecycle

Fixed Groovy syntax for shutdown hook (closure syntax works, Java Thread syntax failed):
- Changed from 'new Thread({...} as Runnable)' to simple closure 'Runtime.addShutdownHook {...}'

All 224 tests pass.

Author: adesjardin

AGENTS.md

@@ -176,9 +176,17 @@ Parent POMs are resolved using embedded Maven Resolver (Eclipse Aether):
 **Backward Compatibility:**
 - Existing methods (`getPomProperty()`, `getDependency()`, `getPlugin()`) still work
 - Parent resolution is explicit - call `pomFile.resolveParents(ParentPomResolver.getInstance())` to enable inheritance
-- Parent resolution failures are logged as warnings (not fatal)
+- `PomFile.resolveParents()` throws `ParentPomResolutionException` on failure (fail-fast)
+- `MuleApplication` catches resolution exceptions and logs warnings, allowing rules to operate on raw POM data
 
-**System Properties:**
+**Resolver Architecture:**
+- `ParentPomResolver.getInstance()` returns a shared singleton instance
+- The singleton is initialized lazily on first use (~500ms startup cost)
+- A JVM shutdown hook automatically calls `close()` on the singleton to release resources
+- Do NOT call `close()` on the singleton instance from application code
+- For tests or custom scenarios, use `new ParentPomResolver(path)` and manage lifecycle yourself
+
+**System Properties:
 - `mule.linter.localRepo`: Custom local repository path (default: ~/.m2/repository)
 
 **Environment:**

mule-linter-spi/src/main/groovy/com/avioconsulting/mule/linter/resolver/ParentPomResolver.groovy

@@ -39,6 +39,14 @@ class ParentPomResolver {
     // Shared instance holder for lazy initialization
     private static class Holder {
         static final ParentPomResolver INSTANCE = new ParentPomResolver()
+        
+        static {
+            // Register shutdown hook to clean up resources when JVM exits
+            // This ensures repositorySystem.shutdown() is called exactly once
+            Runtime.addShutdownHook {
+                INSTANCE.close()
+            }
+        }
     }
 
     /**
@@ -190,8 +198,14 @@ class ParentPomResolver {
 
     /**
      * Shuts down the resolver and cleans up resources.
-     * Call this when the application is exiting to release HTTP connections,
-     * thread pools, and other resources held by Maven Resolver.
+     * 
+     * IMPORTANT: For the singleton instance (getInstance()), this is called
+     * automatically via JVM shutdown hook. Do NOT call close() on the singleton
+     * from individual MuleApplication instances.
+     * 
+     * For manually created instances (new ParentPomResolver()), call this
+     * when the instance is no longer needed to release HTTP connections,
+     * thread pools, and other resources.
      */
     void close() {
         // RepositorySystem manages HTTP clients, thread pools, etc.