chore: add Claude Code configuration

- add CLAUDE.md (project description, architecture, domain notes, git workflow)
- add shared .claude/settings.json (allowed commands, .env read protection)
- pin Prisma agent skills in skills-lock.json and document how to restore them
- ignore generated skills, local settings and Postman skills in .gitignore
- add .gitattributes to normalize line endings to LF

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -31,6 +31,14 @@
 $ npm install
 ```
 
+### AI agent skills (optional)
+
+The Prisma skills used by Claude Code are pinned in `skills-lock.json`; the skill folders themselves are not committed. Restore them with:
+
+```bash
+$ npx skills experimental_install
+```
+
 ## Compile and run the project
 
 ```bash