Sync_with_hub cleanup wipes the whole hub_ops/ cache instead of the completed operation's files

[nklyy]

hub_ops/ is a wallet-wide flat download cache for hub files: files of all operations live side by side, filename = file_id (src/wallet/multisig.rs:1148-1170, tag v0.3.0-beta.28; same on dev). get_or_download_file checks the cache first and only then downloads from the hub.

The problem is in the cleanup step of sync_with_hub (src/wallet/multisig.rs:1938-1945): when the just-processed operation reaches Approved or Discarded, the code removes every file in the directory (read_dir + remove_file over the whole hub_ops/), not just the files belonging to that operation. Cached files of other pending operations disappear until they are re-downloaded.

// cleanup cache for approved or discarded operations
if op.status == OperationStatus::Approved || op.status == OperationStatus::Discarded {
    let ops_dir = self.get_hub_ops_dir();
    if ops_dir.exists() {
        for entry in fs::read_dir(&ops_dir)? {
            fs::remove_file(entry?.path())?;
        }
    }
}

Impact:

• Race with concurrent readers. A long-lived service holds one wallet dir across several loops (sign, completion, sync). If one flow has already resolved a file path (get_or_download_files returns filepath) and a neighbouring sync_with_hub completes an unrelated operation at that moment, the file vanishes from under the reader: reading the PSBT or consignment fails mid-signing.
• Unnecessary hub dependency at signing time. Even without the race, every pending operation's files must be re-downloaded; if the hub is unreachable at that moment, an operation whose files were already local is blocked.

Proposed fix (semantics unchanged, only "cleaning up someone else's files" goes away) - either:

1. delete only the completed operation's files - OperationResponse.files already carries their file_ids, so the loop becomes for file in &op.files { remove get_cached_file_path(&file.file_id) }; or
2. key the cache by subdirectory per operation_idx and remove that subdirectory.