feat(02-03): implement PromptLoader class

- PromptLoader loads and renders markdown templates
- get() returns raw template content (cached)
- render() substitutes {{variable}} placeholders
- Missing variables left as-is (safe substitution)
- Default template_dir points to prompts/ directory

All 10 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gsd-openclaw/src/gsd_openclaw/prompt_loader.py

@@ -0,0 +1,65 @@
+"""Prompt template loader for GSD-OpenClaw."""
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Dict, Any
+
+
+class PromptLoader:
+    """Load and render prompt templates with {{variable}} substitution.
+
+    Uses simple string replacement for {{variable}} syntax.
+    Missing variables are left as-is (safe substitution behavior).
+    """
+
+    def __init__(self, template_dir: Path | None = None):
+        """Initialize PromptLoader.
+
+        Args:
+            template_dir: Directory containing template files.
+                         Defaults to src/gsd_openclaw/prompts/.
+        """
+        if template_dir is None:
+            template_dir = Path(__file__).parent / "prompts"
+        self.template_dir = Path(template_dir)
+        self._cache: Dict[str, str] = {}
+
+    def get(self, name: str) -> str:
+        """Get raw template content by name (cached).
+
+        Args:
+            name: Template name without .md extension.
+
+        Returns:
+            Raw template content.
+
+        Raises:
+            FileNotFoundError: If template file doesn't exist.
+        """
+        if name not in self._cache:
+            path = self.template_dir / f"{name}.md"
+            if not path.exists():
+                raise FileNotFoundError(f"Template not found: {name}")
+            self._cache[name] = path.read_text(encoding='utf-8')
+        return self._cache[name]
+
+    def render(self, name: str, **variables: Any) -> str:
+        """Render a template with variable substitution.
+
+        Uses {{variable}} syntax. Missing variables are left as literal
+        {{variable}} strings (safe substitution).
+
+        Args:
+            name: Template name without .md extension.
+            **variables: Variable values to substitute.
+
+        Returns:
+            Rendered template content.
+
+        Raises:
+            FileNotFoundError: If template file doesn't exist.
+        """
+        content = self.get(name)
+        for key, value in variables.items():
+            content = content.replace("{{" + key + "}}", str(value))
+        return content