Mark old 1.0.0 install note as superseded; add flat-layout CI guard

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C7iVdviBb4QhpAT1UsuyHV

README.md

@@ -36,6 +36,8 @@ Read the full release notes: [v1.1.0](https://github.com/hamen/material-3-skill/
 - **Portable skill layout**: the skill now lives at `skills/material-3/SKILL.md`, with references under `skills/material-3/references/`.
 - **Plugin manifest**: `.claude-plugin/plugin.json` declares the `material-3` plugin metadata for Claude Code while keeping the skill content usable by other skill-aware assistants.
 
+> **Superseded in 1.1.1.** The `claude plugin install github:…` command above never worked and the plugin manifest has moved — use the current commands in [Installation](#installation).
+
 Read the full release notes: [v1.0.0](https://github.com/hamen/material-3-skill/releases/tag/v1.0.0).
 
 ---

bin/ci

@@ -18,6 +18,12 @@ if [[ ! -f "$plugin_path/.claude-plugin/plugin.json" ]]; then
   exit 1
 fi
 
+# Guard against the wrong nested layout (skills/<plugin>/skills/...).
+if [[ -d "$plugin_path/skills" ]]; then
+  printf 'unexpected nested skills/ directory inside %s\n' "$plugin_path" >&2
+  exit 1
+fi
+
 # Strict schema validation when the Claude Code CLI is available.
 if command -v claude >/dev/null 2>&1; then
   claude plugin validate --strict .claude-plugin/marketplace.json