fix: resolve custom instruction paths relative to config file

[dl-alexandre]

Issue #4758

Added splitGlob utility function to split a path into a safe base directory and a glob pattern. This handles cases where Bun.Glob fails with recursive globs in the current working directory.

investigated simplifying the solution by using Bun.Glob with cwd: "/", but it proved unreliable for this specific use case. The splitGlob approach is more robust.