feat(activities): resolve_context stub + evaluate_templates — T15/T16

activities.py — resolve_context (T15):
  - dispatches on source.type: 'static' returns config["value"]
  - 'http_get' / 'db_query' raise ApplicationError(non_retryable=True)
  - unknown types raise ApplicationError(non_retryable=True)

template_engine.py — evaluate_templates (T16, pure function):
  - evaluates optional condition expressions against context snapshot
    (restricted eval, no builtins)
  - interpolates {context.<name>.<key>} placeholders via str.format_map
  - returns list[{task_type, params}] with falsy-condition rows omitted

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

src/activity_core/activities.py

@@ -83,11 +83,35 @@ async def load_activity_definition(activity_id: str) -> dict:
 
 @activity.defn
 async def resolve_context(context_sources: list[dict]) -> dict:
-    """Fetch and merge all context sources into a single snapshot dict.
+    """Resolve each context source and merge into a snapshot dict.
 
-    Implemented in T15.
+    Returns: {source.name: resolved_value, ...}
+
+    Supported source types:
+      static   — returns config["value"] directly
+      http_get — not yet implemented
+      db_query — not yet implemented
     """
-    raise NotImplementedError("T15")
+    snapshot: dict = {}
+    for source in context_sources:
+        name = source["name"]
+        source_type = source["type"]
+        config = source.get("config", {})
+
+        if source_type == "static":
+            snapshot[name] = config.get("value")
+        elif source_type in ("http_get", "db_query"):
+            raise ApplicationError(
+                f"Context source type {source_type!r} is not yet implemented",
+                non_retryable=True,
+            )
+        else:
+            raise ApplicationError(
+                f"Unknown context source type {source_type!r}",
+                non_retryable=True,
+            )
+
+    return snapshot
 
 
 @activity.defn