fix: also skip plan_tool from deterministic pruning

Research and plan_tool outputs are critical for task continuity
across compaction — they should never be truncated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent/context_manager/manager.py

@@ -222,7 +222,7 @@ class ContextManager:
         return False
 
     # Tools whose outputs should never be pruned (too valuable to summarise)
-    _PRUNE_SKIP_TOOLS: set[str] = {"research"}
+    _PRUNE_SKIP_TOOLS: set[str] = {"research", "plan_tool"}
 
     def prune_old_tool_outputs(self) -> None:
         """Stage 1 compaction: deterministically truncate old tool outputs.