Use underscores instead of dashes for AI-generated filenames (#40)

Updated the artifact filename generator to use snake_case (underscores)
instead of kebab-case (dashes) for better consistency with Python
naming conventions.

Changes:
- Updated AI prompt examples to show underscore format
- Modified filename processing to replace spaces with underscores
- Updated validation regex to accept underscores instead of dashes
- Changed docstring to reflect underscore usage

Examples: hello_world, prime_checker, array_sort (instead of hello-world, etc.)

Co-authored-by: Claude <noreply@anthropic.com>

app.py

@@ -335,7 +335,7 @@ def get_activities():
 def generate_artifact_name():
     """Generate a meaningful filename for an artifact using AI.
 
-    Returns a 1-3 word filename with dashes based on what the code does.
+    Returns a 1-3 word filename with underscores based on what the code does.
     Respects ENABLE_CODE_GEN_FILENAMES environment variable (enabled by default).
     """
     # Check if feature is enabled (default: true)
@@ -359,16 +359,16 @@ def generate_artifact_name():
 Rules:
 - Output ONLY the filename, nothing else
 - Use 1-3 words maximum
-- Use lowercase with dashes between words (e.g., "fizzbuzz" or "hello-world" or "prime-checker")
+- Use lowercase with underscores between words (e.g., "fizzbuzz" or "hello_world" or "prime_checker")
 - NO file extension
 - NO explanations or commentary
 - Be specific about what the code does
 
 Examples:
-- Code that prints "Hello World" → "hello-world"
-- Code that checks for prime numbers → "prime-checker"
+- Code that prints "Hello World" → "hello_world"
+- Code that checks for prime numbers → "prime_checker"
 - Code that plays FizzBuzz → "fizzbuzz"
-- Code that sorts an array → "array-sort"
+- Code that sorts an array → "array_sort"
 - Code that calculates factorial → "factorial"
 """
 
@@ -389,12 +389,12 @@ Examples:
         # Clean up the filename (remove quotes, extensions, whitespace)
         filename = filename.strip('"\'')
         filename = filename.split('.')[0]  # Remove any extension
-        filename = filename.replace(' ', '-')
+        filename = filename.replace(' ', '_')
         filename = filename.lower()
 
-        # Validate filename (alphanumeric and dashes only)
+        # Validate filename (alphanumeric and underscores only)
         import re
-        if not re.match(r'^[a-z0-9-]+$', filename):
+        if not re.match(r'^[a-z0-9_]+$', filename):
             filename = "compiled_binary"
 
         # Ensure it's not too long (max 50 chars)