fix(docs): match README manual-uninstall snippet to install layout

[mvanhorn]

Summary

The manual-removal block in README.md told users to run:

find ~/.claude/skills -maxdepth 1 -type l 2>/dev/null | while read -r link; do
  case "$(readlink "$link" 2>/dev/null)" in gstack/*|*/gstack/*) rm -f "$link" ;; esac
done

setup (lines 399-404) creates each skill as a real directory at the top level with SKILL.md as a nested symlink. -maxdepth 1 -type l only matches top-level symlinks, so this find matched nothing on current installs and users were left with ~30 empty per-skill directories.

Fix

Replace the snippet with one that iterates real directories, skips gstack itself, checks the nested SKILL.md is a symlink pointing into gstack/, and removes the directory if so.

find ~/.claude/skills -mindepth 1 -maxdepth 1 -type d 2>/dev/null | while IFS= read -r dir; do
  [ "$(basename "$dir")" = "gstack" ] && continue
  link="$dir/SKILL.md"
  [ -L "$link" ] || continue
  case "$(readlink "$link" 2>/dev/null)" in gstack/*|*/gstack/*) rm -rf "$dir" ;; esac
done

Verified on a fake HOME:

BEFORE:  ~/.claude/skills/{autoplan, ship, gstack, other-tool}
AFTER:   ~/.claude/skills/{gstack, other-tool}
         (autoplan and ship removed; gstack untouched for step 3; other-tool preserved)

Sibling fix for the same root cause in bin/gstack-uninstall is in #902.

Fixes #1130

[mvanhorn]

Appreciate the review @dgrant. Glad the rewritten find pipeline reads right after the install-layout shift in #1130.