docs: fix experimental config key typo in README examples (#329)

Fix dcp_on_compaction_failure → dcp_for_compaction in JSON examples
to match actual schema and code implementation.

Cherry-picked from #325 (merged to master instead of dev)

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

README.md

@@ -959,7 +959,7 @@ Opt-in experimental features that may change or be removed in future versions. U
     "aggressive_truncation": true,
     "auto_resume": true,
     "truncate_all_tool_outputs": false,
-    "dcp_on_compaction_failure": true
+    "dcp_for_compaction": true
   }
 }
 ```