fix(tracing): use Langfuse compile_prompt with {{variable}} syntax

- tracing.py: add compile_prompt() that uses Langfuse .compile(**vars)
  for {{variable}} substitution, falls back to Python .format() for
  hardcoded {variable} templates
- agent_runner.py: replace _get_system_prompt().format() with
  tracing.compile_prompt() for batch_file_classifier, batch_processing,
  batch_cloud_processing prompts
- journey.py: replace get_prompt + .format() with compile_prompt()
  for journey_system prompt
- chat tracing.py: add compile_prompt() for parity (chat prompts
  currently have no variables, but ready for future use)
- Remove unused _get_system_prompt helper

services/batch-agent/app/agent_runner.py

@@ -167,12 +167,6 @@ and what you created.
 """
 
 
-def _get_system_prompt(langfuse_name: str, fallback: str) -> str:
-    """Fetch a managed prompt from Langfuse, falling back to the hardcoded string."""
-    managed = tracing.get_prompt(langfuse_name, fallback=None)
-    return managed if managed is not None else fallback
-
-
 # ── LLM tool-calling loop ─────────────────────────────────────────────────
 
 
@@ -427,9 +421,13 @@ async def _classify_file(
         if d in _DOMAIN_DESCRIPTIONS
     )
 
-    system = _get_system_prompt("batch_file_classifier", _STEP1_SYSTEM_PROMPT).format(
-        domain_definitions=domain_definitions,
-        projects_list=projects_list,
+    system = tracing.compile_prompt(
+        "batch_file_classifier",
+        fallback=_STEP1_SYSTEM_PROMPT,
+        variables={
+            "domain_definitions": domain_definitions,
+            "projects_list": projects_list,
+        },
     )
 
     llm = get_llm(callbacks=[langfuse_handler] if langfuse_handler else None)
@@ -604,13 +602,15 @@ async def run_local_agent(user_id: str, trigger_data: dict[str, Any], *, langfus
 
                 existing_context = "\n\n".join(existing_blocks)
 
-                system_prompt = _get_system_prompt(
-                    "batch_processing", _PROCESSING_SYSTEM_PROMPT
-                ).format(
-                    existing_context=existing_context,
-                    project_context=project_context,
-                    data_types=", ".join(domains),
-                    custom_prompt_section=custom_section,
+                system_prompt = tracing.compile_prompt(
+                    "batch_processing",
+                    fallback=_PROCESSING_SYSTEM_PROMPT,
+                    variables={
+                        "existing_context": existing_context,
+                        "project_context": project_context,
+                        "data_types": ", ".join(domains),
+                        "custom_prompt_section": custom_section,
+                    },
                 )
 
                 processing_tools = _build_processing_tools(domains)
@@ -790,13 +790,15 @@ async def run_cloud_agent(user_id: str, config_id: str, *, langfuse_handler: Any
                 continue
             items_processed += 1
 
-            processing_prompt = _get_system_prompt(
-                "batch_cloud_processing", _CLOUD_PROCESSING_PROMPT
-            ).format(
-                data_types=", ".join(config.data_types),
-                project_context="Determine the appropriate project from the message context.",
-                file_list=f"Message from {config.provider} (id: {msg.id})",
-                custom_prompt_section=custom_section,
+            processing_prompt = tracing.compile_prompt(
+                "batch_cloud_processing",
+                fallback=_CLOUD_PROCESSING_PROMPT,
+                variables={
+                    "data_types": ", ".join(config.data_types),
+                    "project_context": "Determine the appropriate project from the message context.",
+                    "file_list": f"Message from {config.provider} (id: {msg.id})",
+                    "custom_prompt_section": custom_section,
+                },
             )
 
             try: