perf(redaction): cache _internal_roots() to avoid recomputation per call

_internal_roots resolves 8 candidate paths + builds 3 separator variants
+ sorts on every redact_internal_paths call. On the worker hot path this
runs once per task error / event / file-tool failure. Result is process-
stable (paths don't change without a restart), so @cache it.

Follow-up to #119.

Author: warren618

agent/src/tools/redaction.py

@@ -11,11 +11,13 @@
 
 import sys
 import sysconfig
+from functools import cache
 from pathlib import Path
 
 _SENTINEL = "<redacted>"
 
 
+@cache
 def _internal_roots() -> list[str]:
     # AGENT_DIR derived like agent/src/providers/llm.py:90
     # (Path(__file__).resolve().parents[2]); redaction.py is agent/src/tools/