Refactor Issue Filename Convention

To create a more robust and scalable file naming convention, the synchronization process will be updated. This involves two key changes:

1. Removing Numeric Padding: The current method of padding issue numbers (e.g., 0123.md) imposes an arbitrary limit on the number of issues the project can have. This will be removed.
2. Adding a Prefix: To avoid starting filenames with a number, which can be problematic for some shell tools, a configurable prefix (e.g., issue-) will be added.

The resulting convention (e.g., issue-123.md) is scalable, robust, and remains human-readable.

Acceptance Criteria

1. A new issueFilenamePrefix property is added to the githubWorkflow.issueSync object in config.mjs with a default value of 'issue-.
2. The issueNumberPadding property is removed from the configuration.
3. The #getIssuePath method in SyncService.mjs is updated to construct filenames using the prefix and the raw, un-padded issue number.
4. The sync logic must gracefully handle the transition. On the next pull, it should detect and rename any existing files that use the old padded-number format to the new prefixed format.