fix: skip non-directory entries in listScopeIds

[yulong-ge]

Problem

listScopeIds() treats every entry returned by readdirSync as a scope ID. On macOS, Finder writes .DS_Store files into the scopes root; any plain file that happens to share the directory would also be mis-identified. This causes downstream operations (job loading, storage init) to fail or produce unexpected results.

Changes

• listScopeIds(): Use readdirSync(root, { withFileTypes: true }) and filter by entry.isDirectory() instead of probing each name with existsSync(scopeDir(name)).
• ensureScopeStorage(): Add a guard — if the resolved scope path exists but is not a directory, return early instead of trying to create sub-dirs inside a file.
• Export listScopeIds with an optional root parameter so it can be unit-tested against temp directories.
• Add unit tests covering: missing dir, empty dir, normal scopes, .DS_Store, stray files, mixed entries, no-throw guarantee.