fix: add encoding="utf-8" to prompt file open() calls in script_runner (Windows CP950) (#607)

PromptCompiler.compile() and _resolve_prompt_file() open .prompt.md files
with plain open() which defaults to the system locale encoding. On Windows
systems set to CP950/CP936/CP932 (Chinese/Japanese/Korean), a UTF-8 encoded
prompt file containing any multibyte character causes:

  UnicodeDecodeError: 'cp950' codec can't decode byte 0x8b in position 12

Three open() calls were missing explicit encoding:
- compiled_path read in ScriptRunner._execute_script
- prompt_path read in PromptCompiler.compile
- output_path write in PromptCompiler.compile

Adding encoding="utf-8" to all three matches the behaviour of the rest of
the codebase and fixes the crash for any non-UTF-8 Windows locale.

Co-authored-by: Sergio Sisternes <sergio_sisternes@epam.com>

Author: edenfunf

src/apm_cli/core/script_runner.py

@@ -256,7 +256,7 @@ def _auto_compile_prompts(
             compiled_prompt_files.append(prompt_file)
 
             # Read the compiled content
-            with open(compiled_path, "r") as f:
+            with open(compiled_path, "r", encoding="utf-8") as f:
                 compiled_content = f.read().strip()
 
             # Check if this is a runtime command (copilot, codex, llm) before transformation
@@ -916,7 +916,7 @@ def compile(self, prompt_file: str, params: Dict[str, str]) -> str:
         # Now ensure compiled directory exists
         self.compiled_dir.mkdir(parents=True, exist_ok=True)
 
-        with open(prompt_path, "r") as f:
+        with open(prompt_path, "r", encoding="utf-8") as f:
             content = f.read()
 
         # Parse frontmatter and content
@@ -939,7 +939,7 @@ def compile(self, prompt_file: str, params: Dict[str, str]) -> str:
         output_path = self.compiled_dir / output_name
 
         # Write compiled content
-        with open(output_path, "w") as f:
+        with open(output_path, "w", encoding="utf-8") as f:
             f.write(compiled_content)
 
         return str(output_path)