feat(config): add {env:VAR} syntax for environment variable references

Add secret_resolver module to resolve {env:VAR_NAME} placeholders in
config files. Allows storing sensitive values like API keys in
environment variables instead of config.json.

nanobot/config/loader.py

@@ -4,6 +4,7 @@ import json
 from pathlib import Path
 
 from nanobot.config.schema import Config
+from nanobot.config.secret_resolver import resolve_config
 
 
 # Global variable to store current config path (for multi-instance support)
@@ -40,6 +41,7 @@ def load_config(config_path: Path | None = None) -> Config:
             with open(path, encoding="utf-8") as f:
                 data = json.load(f)
             data = _migrate_config(data)
+            data = resolve_config(data)  # Resolve {env:VAR} references
             return Config.model_validate(data)
         except (json.JSONDecodeError, ValueError) as e:
             print(f"Warning: Failed to load config from {path}: {e}")

nanobot/config/secret_resolver.py

@@ -0,0 +1,48 @@
+"""Secret reference resolver for configuration.
+
+Supports {env:VARIABLE_NAME} syntax to reference environment variables.
+"""
+
+import os
+import re
+
+# Pattern matches {env:VAR_NAME} where VAR_NAME follows env var naming conventions
+_REF_PATTERN = re.compile(r"\{env:([A-Z_][A-Z0-9_]*)\}")
+
+
+def resolve_env_vars(value: str) -> str:
+    """Resolve {env:VAR} references in a string.
+
+    Args:
+        value: String that may contain {env:VAR} references.
+
+    Returns:
+        String with all {env:VAR} references replaced by their values.
+        Unresolved references are left unchanged.
+    """
+    def replacer(match: re.Match[str]) -> str:
+        var_name = match.group(1)
+        env_value = os.environ.get(var_name)
+        if env_value is None:
+            return match.group(0)  # Keep original if env var doesn't exist
+        return env_value
+
+    return _REF_PATTERN.sub(replacer, value)
+
+
+def resolve_config(obj):
+    """Recursively resolve {env:VAR} references in a configuration object.
+
+    Args:
+        obj: Configuration value (str, dict, list, or other).
+
+    Returns:
+        Configuration with all {env:VAR} references resolved.
+    """
+    if isinstance(obj, str):
+        return resolve_env_vars(obj)
+    if isinstance(obj, dict):
+        return {k: resolve_config(v) for k, v in obj.items()}
+    if isinstance(obj, list):
+        return [resolve_config(item) for item in obj]
+    return obj