fix: Update tests to match schema reference metadata feature

Fix failing tests that expected content to start with heading but now
include schema reference comments. Also fix validate command syntax
in test (positional to --schema flag).

Fixes:
- test_generate_stub_with_explicit_associated_path
- test_generate_stub_interactive_mode_defaults_to_associated_path
- test_generate_stub_validates_generated_draft_against_schema

These tests were failing due to changes from Issue #55 schema reference
metadata feature and validate command syntax updates.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

tests/test_issue_55_schema_based_draft_generation.py

@@ -382,7 +382,7 @@ class TestIssue55SchemaBasedDraftGeneration:
             validate_result = self.runner.invoke(cli, [
                 'validate',
                 str(draft_file),
-                str(schema_file)
+                '--schema', str(schema_file)
             ])
 
             # Assert - Generated draft should be valid against its source schema