Add a `paths` option to `memorySearch` config, allowing users to
explicitly specify additional directories or files to include in
memory search.
Follow-up to #2961 as suggested by @gumadeiras — instead of auto-following
symlinks (which has security implications), users can now explicitly
declare additional search paths.
- Add `memorySearch.paths` config option (array of strings)
- Paths can be absolute or relative (resolved from workspace)
- Directories are recursively scanned for `.md` files
- Single `.md` files can also be specified
- Paths from defaults and agent overrides are merged
- Added 4 test cases for listMemoryFiles
