extras: make mksboarchive resolve the package from CWD

The previous version derived the repo root from the script's own location via
git, which broke once the script was installed outside the repo (e.g. ~/bin):
it ran git in ~/bin and failed with 'not a git repository'. Resolve the
package as <pkg>/ below the current directory instead and run from the repo
root. The installed copy now works identically. README updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/README.md b/README.md
index 207afc6..f2142c2 100644
--- a/README.md
+++ b/README.md
@@ -73,11 +73,13 @@ rather than at commit time:
 OUTPUT=/tmp .extras/mksboarchive hydra   # writes /tmp/hydra.tar.gz
 ```
 
-It validates that `<package>/<package>.SlackBuild` exists, removes any
-`sbodl` source symlinks from the package directory first (so they never leak
-into the tarball), then archives the directory. The output directory defaults
-to the current working directory and can be overridden with the `OUTPUT`
-environment variable. Run it from anywhere; it locates the repo root via git.
+Run it from the repo root: it looks for a `<package>/<package>.SlackBuild`
+below the current directory, removes any `sbodl` source symlinks from the
+package directory first (so they never leak into the tarball), then archives
+the directory. The output directory defaults to the current working directory
+and can be overridden with the `OUTPUT` environment variable. This means it
+works the same whether you run `.extras/mksboarchive` or an installed copy
+(e.g. in `~/bin`), as long as you are in the repo root.
 
 ## Vendored Crates (Rust packages)