Add .claude/, .gitignore, and update README

- .claude/settings.json: project-level Claude Code config
- .gitignore: ignores test binary, build artifacts, shards cache, tags
- README.md: expanded with usage example, dev commands, and AI assistance
  disclosure (Claude used for review/bug-checking, not code generation)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ATnpoEGXj4RyuMoBLGWcjs

.gitignore

@@ -0,0 +1,14 @@
+# Crystal build artifacts
+/.build/
+/bin/
+
+# Compiled test binary (crystal build test.cr)
+test
+
+# Shards cache
+/.shards/
+/lib/
+
+# Editor
+tags
+.DS_Store