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/journey.py

@@ -145,15 +145,17 @@ def _build_system_prompt(
         if existing_template
         else ""
     )
-    # Try Langfuse-managed prompt first, fall back to hardcoded template
-    managed = tracing.get_prompt("journey_system", fallback=None)
-    template = managed if managed is not None else _SYSTEM_PROMPT_TEMPLATE
-    return template.format(
-        directory=directory,
-        data_types=", ".join(data_types),
-        template_start=_TEMPLATE_START,
-        template_end=_TEMPLATE_END,
-        existing_section=existing_section,
+    # Use Langfuse compile_prompt ({{variable}} syntax) with Python .format() fallback
+    return tracing.compile_prompt(
+        "journey_system",
+        fallback=_SYSTEM_PROMPT_TEMPLATE,
+        variables={
+            "directory": directory,
+            "data_types": ", ".join(data_types),
+            "template_start": _TEMPLATE_START,
+            "template_end": _TEMPLATE_END,
+            "existing_section": existing_section,
+        },
     )