feat: Add system-prompt-file input for file-based system prompts

This enhancement adds the ability to load a system prompt from a file, similar to
the existing prompt-file functionality, providing more flexibility when working with
complex system prompts.

Key changes:
- Added new `system-prompt-file` input to action.yml with proper description
- Updated main.ts implementation to handle file-based system prompts with:
  - File existence checking and appropriate error handling
  - Proper precedence (system-prompt-file takes priority over system-prompt)
  - Consistent error messages with existing prompt-file implementation

Test coverage added:
- Basic usage: Test verifies system-prompt-file loads content correctly
- Error handling: Test ensures proper errors when system-prompt-file doesn't exist
- Precedence: Test confirms system-prompt-file overrides system-prompt when both exist
- Integration: Test validates both prompt-file and system-prompt-file work together
- Max tokens: Test verifies custom token limits are properly passed to the model
- Testing infrastructure: Improved mock implementations that detect unexpected calls

Documentation:
- Updated README.md with system-prompt-file in inputs table
- Added dedicated usage example for system-prompt-file
- Fixed formatting in inputs table

CI/CD:
- Updated workflow to test system-prompt-file functionality with actual file content

This feature maintains backward compatibility while adding a useful option
for workflows that need to use more complex system prompts stored in files.

src/main.ts

@@ -29,7 +29,22 @@ export async function run(): Promise<void> {
       throw new Error('prompt is not set')
     }
 
-    const systemPrompt: string = core.getInput('system-prompt')
+    const systemPromptFile: string = core.getInput('system-prompt-file')
+    const systemPromptString: string = core.getInput('system-prompt')
+
+    let systemPrompt: string
+    if (systemPromptFile !== undefined && systemPromptFile !== '') {
+      if (!fs.existsSync(systemPromptFile)) {
+        throw new Error(`System prompt file not found: ${systemPromptFile}`)
+      }
+      systemPrompt = fs.readFileSync(systemPromptFile, 'utf-8')
+    } else if (systemPromptString !== undefined && systemPromptString !== '') {
+      systemPrompt = systemPromptString
+    } else {
+      // Use default system prompt
+      systemPrompt = 'You are a helpful assistant'
+    }
+
     const modelName: string = core.getInput('model')
     const maxTokens: number = parseInt(core.getInput('max-tokens'), 10)