fix: allow env overrides to create missing config keys (#2064)

Previously, environment variable overrides (e.g. SYSTEM__INSTANCE_ID)
were silently skipped if the target key didn't already exist in
data/config.yaml. This caused SaaS pods running older LangBot images
(whose config template lacked system.instance_id) to ignore the
SYSTEM__INSTANCE_ID env var, falling back to a random UUID that
didn't match the pod UUID — breaking idle timeout tracking.

Now env overrides create missing keys (as strings) and missing
intermediate dicts, so they work regardless of template version.

Co-authored-by: rocksclawbot <rocksclawbot@users.noreply.github.com>

src/langbot/pkg/core/stages/load_config.py

@@ -74,11 +74,12 @@ def _apply_env_overrides_to_config(cfg: dict) -> dict:
         current = cfg
 
         for i, key in enumerate(keys):
-            if not isinstance(current, dict) or key not in current:
+            if not isinstance(current, dict):
                 break
 
             if i == len(keys) - 1:
-                # At the final key - check if it's a scalar value
+                # At the final key
+                if key in current:
                     if isinstance(current[key], (dict, list)):
                         # Skip dict and list types
                         pass
@@ -87,7 +88,12 @@ def _apply_env_overrides_to_config(cfg: dict) -> dict:
                         converted_value = convert_value(env_value, current[key])
                         current[key] = converted_value
                 else:
-                # Navigate deeper
+                    # Key doesn't exist yet - create it as string
+                    current[key] = env_value
+            else:
+                # Navigate deeper - create intermediate dict if needed
+                if key not in current:
+                    current[key] = {}
                 current = current[key]
 
     return cfg