feat: per-client Makefile infrastructure for 4-mode testing

Add per-client Makefiles for C, Python, and Go with:
- CLI mode: Tests --help, arg parsing, syntax validation
- Library mode: Unit tests, import verification
- Integration mode: API contract validation (with credentials)
- Functional mode: Real-world scenario tests

C client:
- 22 library tests (SHA-256, HMAC-SHA256, detect_language)
- Full unsandbox.c implementation with examples

Python client (sync + async):
- Delegates to sync/ and async/ subdirectories
- pytest-based test suites with coverage
- Examples for concurrent execution, streaming

Go client:
- Delegates to sync/ and async/ subdirectories
- go test integration with vet and fmt

Also update detect-changes.sh to detect changes in
both root-level un.* files AND clients/ directory.

clients/python/sync/examples/json_processing.py

@@ -0,0 +1,100 @@
+#!/usr/bin/env python3
+"""
+JSON Processing example for unsandbox Python SDK - Synchronous Version
+
+This example demonstrates JSON parsing and manipulation operations.
+Shows how to work with structured data in sandboxed environments.
+
+To run:
+    export UNSANDBOX_PUBLIC_KEY="your-public-key"
+    export UNSANDBOX_SECRET_KEY="your-secret-key"
+    python3 json_processing.py
+
+Expected output:
+    Original JSON: {"name": "Alice", "age": 30, "skills": ["Python", "JavaScript"]}
+    Parsed successfully!
+    Name: Alice
+    Age: 30
+    Skills: Python, JavaScript
+"""
+
+import sys
+import os
+
+# Add the SDK path
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "src"))
+
+from un import execute_code, CredentialsError
+
+
+def main():
+    """Execute JSON processing code in sandbox."""
+
+    # The code to execute - demonstrates JSON parsing and manipulation
+    code = """
+import json
+
+# Original JSON data
+json_string = '{"name": "Alice", "age": 30, "skills": ["Python", "JavaScript"]}'
+print(f"Original JSON: {json_string}")
+
+try:
+    # Parse JSON
+    data = json.loads(json_string)
+    print("Parsed successfully!")
+
+    # Access fields
+    print(f"Name: {data['name']}")
+    print(f"Age: {data['age']}")
+    print(f"Skills: {', '.join(data['skills'])}")
+
+    # Modify and re-serialize
+    data['age'] = 31
+    data['skills'].append("Go")
+    modified_json = json.dumps(data, indent=2)
+    print(f"\\nModified JSON:\\n{modified_json}")
+
+except json.JSONDecodeError as e:
+    print(f"JSON parsing error: {e}")
+except Exception as e:
+    print(f"Error: {e}")
+"""
+
+    try:
+        # Resolve credentials from environment
+        public_key = os.environ.get("UNSANDBOX_PUBLIC_KEY")
+        secret_key = os.environ.get("UNSANDBOX_SECRET_KEY")
+
+        if not public_key or not secret_key:
+            print("Error: UNSANDBOX_PUBLIC_KEY and UNSANDBOX_SECRET_KEY environment variables required")
+            print("Run with: export UNSANDBOX_PUBLIC_KEY=your-key UNSANDBOX_SECRET_KEY=your-key")
+            sys.exit(1)
+
+        # Execute the code
+        print("Executing JSON processing in sandbox...")
+        result = execute_code("python", code, public_key, secret_key)
+
+        # Check for errors
+        if result.get("status") == "completed":
+            print("\n=== STDOUT ===")
+            print(result.get("stdout", ""))
+            if result.get("stderr"):
+                print("\n=== STDERR ===")
+                print(result.get("stderr", ""))
+        else:
+            print(f"Execution failed with status: {result.get('status')}")
+            print(f"Error: {result.get('error', 'Unknown error')}")
+            sys.exit(1)
+
+    except CredentialsError as e:
+        print(f"Credentials error: {e}")
+        sys.exit(1)
+    except Exception as e:
+        print(f"Error: {e}")
+        import traceback
+        traceback.print_exc()
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()