Fix: Echte UTF-8-Umlaute in KI-generierten Inhalten erzwingen

- Claude CLI Umgebung: LANG=C.UTF-8, LC_ALL=C.UTF-8 setzen
- Alle 10 Agent-Prompts: Explizite Anweisung für echte Umlaute (ä,ö,ü,ß)
  statt Umschreibungen (ae,oe,ue,ss)
- Betrifft: Researcher, Analyzer, Factchecker (jeweils initial + inkrementell)

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

src/agents/claude_client.py

@@ -56,7 +56,12 @@ async def call_claude(prompt: str, tools: str | None = "WebSearch,WebFetch", mod
 
     process = await asyncio.create_subprocess_exec(
         *cmd, stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE,
-        env={"PATH": "/usr/local/bin:/usr/bin:/bin", "HOME": "/home/claude-dev"},
+        env={
+            "PATH": "/usr/local/bin:/usr/bin:/bin",
+            "HOME": "/home/claude-dev",
+            "LANG": "C.UTF-8",
+            "LC_ALL": "C.UTF-8",
+        },
     )
     try:
         stdout, stderr = await asyncio.wait_for(process.communicate(), timeout=CLAUDE_TIMEOUT)