skill-creator: sync from anthropics/skills (drop ANTHROPIC_API_KEY requirement) (#1523)

Ports anthropics/skills#547 (b0cbd3d) so this repo matches the upstream
skills repo.

improve_description.py and run_loop.py now shell out to `claude -p` instead
of using the Anthropic SDK directly, so the description optimizer uses the
session's existing Claude Code auth and no longer requires a separate
ANTHROPIC_API_KEY. SKILL.md drops the stale extended-thinking reference and
adds guidance for updating an existing skill.

Several enterprise customers sync exclusively from this repo (not
anthropics/skills, whose README disclaims production use), so they have been
stuck on the old SDK-based path.

plugins/skill-creator/skills/skill-creator/scripts/run_loop.py

@@ -15,8 +15,6 @@ import time
 import webbrowser
 from pathlib import Path
 
-import anthropic
-
 from scripts.generate_report import generate_html
 from scripts.improve_description import improve_description
 from scripts.run_eval import find_project_root, run_eval
@@ -75,7 +73,6 @@ def run_loop(
         train_set = eval_set
         test_set = []
 
-    client = anthropic.Anthropic()
     history = []
     exit_reason = "unknown"
 
@@ -200,7 +197,6 @@ def run_loop(
             for h in history
         ]
         new_description = improve_description(
-            client=client,
             skill_name=name,
             skill_content=content,
             current_description=current_description,