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/http_request.py

@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+"""
+HTTP Request example for unsandbox Python SDK - Synchronous Version
+
+This example demonstrates making HTTP requests from within a sandboxed environment.
+Uses semitrusted mode which provides internet access through an egress proxy.
+
+To run:
+    export UNSANDBOX_PUBLIC_KEY="your-public-key"
+    export UNSANDBOX_SECRET_KEY="your-secret-key"
+    python3 http_request.py
+
+Expected output:
+    Status Code: 200
+    Response: {"origin": "..."}
+"""
+
+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 HTTP request code in sandbox."""
+
+    # The code to execute - uses requests library (pre-installed)
+    code = """
+import requests
+import json
+
+try:
+    # Make HTTP request to httpbin.org
+    response = requests.get('https://httpbin.org/ip', timeout=10)
+    print(f"Status Code: {response.status_code}")
+
+    # Parse and display response
+    data = response.json()
+    print(f"Response: {json.dumps(data)}")
+
+except requests.RequestException as e:
+    print(f"Request failed: {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 HTTP request 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()