fix(tools): memory no-match is success, not error

Fixes #313

Problem: MemoryStore.replace() and .remove() return
{"success": false, "error": "No entry matched..."} when the
search substring is not found. This is a valid outcome, not
an error. The empirical audit showed 58.4% error rate on the
memory tool, but 98.4% of those were just empty search results.

Fix: Return {"success": true, "result": "no_match", "message": ...}
instead. This drops the memory tool error rate from ~58% to ~1%.

Tests updated: test_replace_no_match and test_remove_no_match
now assert success=True with result="no_match".
All 33 memory tool tests pass.

tests/tools/test_memory_tool.py

@@ -144,7 +144,8 @@ class TestMemoryStoreReplace:
     def test_replace_no_match(self, store):
         store.add("memory", "fact A")
         result = store.replace("memory", "nonexistent", "new")
-        assert result["success"] is False
+        assert result["success"] is True
+        assert result["result"] == "no_match"
 
     def test_replace_ambiguous_match(self, store):
         store.add("memory", "server A runs nginx")
@@ -177,7 +178,8 @@ class TestMemoryStoreRemove:
 
     def test_remove_no_match(self, store):
         result = store.remove("memory", "nonexistent")
-        assert result["success"] is False
+        assert result["success"] is True
+        assert result["result"] == "no_match"
 
     def test_remove_empty_old_text(self, store):
         result = store.remove("memory", "  ")