Refa: refactor prompts into markdown-style structure using Jinja2 (#8667)

### What problem does this PR solve?

Refactor prompts into markdown-style structure using Jinja2.

### Type of change

- [x] Refactoring

rag/prompt_template.py

@@ -0,0 +1,21 @@
+import os
+
+BASE_DIR = os.path.dirname(__file__)
+
+PROMPT_DIR = os.path.join(BASE_DIR, "prompts")
+
+_loaded_prompts = {}
+
+
+def load_prompt(name: str) -> str:
+    if name in _loaded_prompts:
+        return _loaded_prompts[name]
+
+    path = os.path.join(PROMPT_DIR, f"{name}.md")
+    if not os.path.isfile(path):
+        raise FileNotFoundError(f"Prompt file '{name}.md' not found in prompts/ directory.")
+
+    with open(path, "r", encoding="utf-8") as f:
+        content = f.read().strip()
+        _loaded_prompts[name] = content
+        return content