fix: address PR review comments

- patch.rs: convert Replace to Add when idx == onto_idx in Remove rebase
- store.rs: use ciborium::from_reader_with_buffer to reuse scratch buffer
- store.rs: compress takes &mut bool instead of returning bool
- store.rs: move OPEN_STORES cleanup to Drop impl (std::sync::Mutex)
- store.rs: PatchDb::close no longer errors on other references
- store.rs: revert exists() to treat null as non-existent
- remove @claude changelog comments from backend

json-ptr/src/lib.rs

@@ -634,11 +634,6 @@ impl JsonPointer<String> {
             PtrSegment::Unescaped(1..prefix_len)
         });
     }
-    // @claude fix #3: Previously just inserted the new segment without adjusting
-    // existing segment ranges (unlike push_start which did shift them). All
-    // existing segments' byte ranges became wrong, causing corrupted pointer
-    // lookups or panics. Now shifts ranges forward by prefix_len, matching
-    // push_start behavior.
     pub fn push_start_idx(&mut self, segment: usize) {
         let mut src = format!("/{}", segment);
         let prefix_len = src.len();