Drop agent tooling files and neutralise spec references

- Removed CLAUDE.md, AGENTS.md and NEXTJS_REBUILD_SUPERPROMPT.md, and
  untracked .claude/ (now ignored locally via .git/info/exclude rather than
  .gitignore, so the repo carries no reference to it either).
- The Next.js 16 warning that lived in AGENTS.md is kept where it is
  actually useful, in the README tech-stack section.
- Source and migration comments referred to "the consolidation master
  prompt" and "NEXTJS_REBUILD_SUPERPROMPT.md" by name; both now read "spec",
  keeping the section numbers that made the cross-references worth having.
- README no longer lists Playwright, which is not installed, and now
  describes the three test layers that actually exist.

supabase/migrations/20260714120600_data_integrity_guards.sql

@@ -1,4 +1,5 @@
--- Two data-integrity rules named explicitly in the spec (§3.7, §11 test scenarios #14 and #17) that had no enforcement
+-- Two data-integrity rules named explicitly in the spec (§3.7, §11 test
+-- scenarios #14 and #17) that had no enforcement
 -- anywhere — not in the UI, not in a Server Action, not as a DB constraint:
 --
 --   1. "Rückkehr darf nicht vor Beginn der Karenz liegen" — there was no