docs(codex-tools): add environment detection and App finishing docs (PRI-823)

Document the git-dir vs git-common-dir detection pattern and the Codex
App's native finishing flow for skills that need to adapt.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/using-superpowers/references/codex-tools.md

@@ -85,7 +85,7 @@ BRANCH=$(git branch --show-current)
 - `BRANCH` empty → detached HEAD (cannot branch/push/PR from sandbox)
 
 See `using-git-worktrees` Step 0 and `finishing-a-development-branch`
-Step 1 for how each skill uses these signals.
+Step 1.5 for how each skill uses these signals.
 
 ## Codex App Finishing