chore: rename CLAUDE.md to AGENTS.md, add symlink and docs/templates

Adds tool-agnostic AGENTS.md as canonical context file, CLAUDE.md symlink for
Claude Code compatibility, and docs/templates/ with spec.md + adr.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/templates/spec.md

@@ -0,0 +1,29 @@
+# [Feature- oder Sprint-Name]
+
+**Status:** Draft | Active | Done
+**Repo:** [repo]
+**Erstellt:** [YYYY-MM-DD]
+
+## Ziel
+
+Ein Satz.
+
+## Warum
+
+Warum jetzt, warum das.
+
+## In Scope
+
+- 
+
+## Out of Scope
+
+- 
+
+## Erfolgskriterien
+
+- [ ] 
+
+## Implementierungsnotizen
+
+<!-- Schlüsselentscheidungen, Constraints, offene Fragen -->