chore: ignore skill install artifacts (#317)

`npx skills add` writes .agents/, .claude/, and skills-lock.json into
consumer projects. Ignore them in this source repo so the install can be
run from inside the repo without polluting git status.

https://claude.ai/code/session_01HStJ5nara1i4H6yrXrio2w

Co-authored-by: Claude <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,11 @@
 # Dependencies
 node_modules/
 
+# Skill install artifacts (npx skills add)
+.agents/
+.claude/
+skills-lock.json
+
 # Environment variables / secrets
 .env
 .env.*