fix(slug): avoid parent repo identity in subdirs

[jbetala7]

Fixes #1125

Summary

• ignore parent repository remotes when gstack-slug is run from a subdirectory that is not the git root
• let a .gstack-slug file in the current directory or an ancestor pin the project slug before cache/git inference
• treat slug cache as a fallback for transient inference failures, not as an immutable source of truth when current inference or an override is available

Root cause

gstack-slug trusted git remote get-url origin before checking whether the current directory was actually the repository root. In a standalone project nested under a larger git repo, Git walked up to the parent repo, so the child project inherited the parent repo slug. Because cache lookup happened first, a bad slug-cache entry could keep that mistake alive indefinitely.

Verification

• bun test test/skill-validation.test.ts -t gstack-slug
• bun test test/review-log.test.ts test/gstack-learnings-search.test.ts
• bun test test/skill-validation.test.ts
• direct repro: nested directory under a parent repo with remote now returns the child basename instead of me-workspace
• git diff --check origin/main...HEAD
• git merge-tree --write-tree HEAD origin/main

Collision check

Searched open PRs/issues for 1125 gstack-slug outer repo subdirectory, gstack-slug subdirectory outer repo, bin/gstack-slug subdirectory slug-cache, slug-cache, and .gstack-slug.

Closest open PR is #564, but that is an older missing-git-context fallback patch. Current origin/main already has basename/unknown fallback behavior; #564 does not address parent-repo inheritance, .gstack-slug overrides, or stale cache recovery.