fix(skills): whats-next creates worktrees under the repo root (.worktrees/), not the parent dir

The parent-dir pattern put worktrees outside the permission sandbox,
breaking agent file access. .gitignore already documented the in-repo
convention; the skill predated it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.claude/skills/whats-next/SKILL.md

@@ -188,9 +188,14 @@ git checkout -b <branch-name>
 
 If the user wants to work in a separate worktree:
 
+Worktrees live **inside the repo** under the gitignored `.worktrees/` directory
+(see `.gitignore` — "created under the repo root"). Never place them in the repo's
+parent directory: paths outside the repo are outside the permission sandbox and
+break agent file access.
+
 ```bash
 REPO_ROOT=$(git rev-parse --show-toplevel)
-WORKTREE_DIR="$(dirname "$REPO_ROOT")/.worktrees/<branch-name>"
+WORKTREE_DIR="$REPO_ROOT/.worktrees/<branch-name>"
 git worktree add "$WORKTREE_DIR" <branch-name>
 ```