[infra] Harden worktree isolation; fix merge script git path

[infra] Harden worktree isolation; fix merge script git path

create-worktree: Block Write/Edit file_path outside the
worktree, Bash cd to main or other worktrees, git -C on main
(except merge-worktree), and shell redirects to main.  Ensure
.claude/skills stays a symlink to main on entry.

merge-worktree, branch-summary: Use plain git from PATH
instead of hardcoded $HOME/.guix-home/profile/bin/git.

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