fix(docs): correct install one-liner raw URL (Gitea, not GitLab) + v0.2.1

git.codelab.vc is Gitea/Forgejo: raw path is /raw/branch/<ref>/, the GitLab-style
/-/raw/main/ returned 404 so the one-command install was broken. The one-liner
now also fetches the script from the stable channel (vetted), not main.

install.sh

@@ -3,7 +3,7 @@
 # pi-kit bootstrap — one-shot setup for new engineers. Idempotent: safe to re-run.
 #
 # Usage:
-#   curl -fsSL https://git.codelab.vc/ai/pi-kit/-/raw/main/install.sh | bash
+#   curl -fsSL https://git.codelab.vc/ai/pi-kit/raw/branch/stable/install.sh | bash
 #   # or, from a checkout:
 #   ./install.sh
 #