The repo_root calculation assumed stack paths are always 4 levels deep
(stack_orchestrator/data/stacks/name). External stacks with different
nesting (e.g. stack-orchestrator/stacks/name = 3 levels) got the wrong
root, causing --spec-file resolution to fail.
Use git rev-parse --show-toplevel instead.
Fixes: so-k1k
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>