fix(effects): make postconditions prove intended mutations

edit_file and apply_patch update claims asserted only existence (or, in the
uncommitted corrective attempt, any content change), so an unrelated write
could verify them. Each filesystem postcondition is now the exact content the
producer's own transformation writes from the identity-checked pre-state:
edit_file through the extracted pure _edit_file_text (no newline
translation), apply_patch updates through _apply_patch_hunks on the
universal-newline pre-state. An oversized, replaced or undecodable pre-state,
a non-matching hunk, or an underivable write_file body leaves the whole claim
without postconditions (UNVERIFIED) instead of letting derivable targets
verify the operation or falling back to existence.

src/agent_tools/filesystem_tools.py

@@ -116,6 +116,20 @@ def _unified_diff(old: str, new: str, path: str) -> Optional[Dict[str, Any]]:
         "file": os.path.basename(path) or (path or "file"),
     }
 
+def _edit_file_text(original: str, old: str, new: str, replace_all: bool) -> tuple[str | None, str]:
+    """The exact text edit_file writes for ``original``, or None and why not.
+
+    Pure: the effect adapter derives the requested post-state from this same
+    function, so the postcondition is the producer's own transformation.
+    """
+    count = original.count(old)
+    if count == 0:
+        return None, "not_found"
+    if count > 1 and not replace_all:
+        return None, f"not_unique:{count}"
+    return (original.replace(old, new) if replace_all else original.replace(old, new, 1)), "ok"
+
+
 class EditFileTool:
     async def execute(self, content: str, ctx: dict) -> dict:
         from src.tool_execution import _resolve_tool_path, _resolve_search_root, _truncate
@@ -150,12 +164,9 @@ class EditFileTool:
             # Exact replacement must not normalize unrelated CRLF/CR newlines.
             with open(path, "r", encoding="utf-8", newline="") as f:
                 original = f.read()
-            count = original.count(old)
-            if count == 0:
-                return original, None, "not_found"
-            if count > 1 and not replace_all:
-                return original, None, f"not_unique:{count}"
-            updated = original.replace(old, new) if replace_all else original.replace(old, new, 1)
+            updated, status = _edit_file_text(original, old, new, replace_all)
+            if updated is None:
+                return original, None, status
             attempted.append(True)
             with open(path, "w", encoding="utf-8", newline="") as f:
                 f.write(updated)