fix(ai): forbid preamble in response-shortener output

Tighten the system/user prompts so the shortener returns ONLY the
rewritten message — no "Here's a version at N characters:" preamble,
no quotes, no labels. Previously such preamble was being transmitted
on-air and consumed the byte budget, causing the actual content to
truncate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

orac/ai.py

@@ -50,10 +50,20 @@ def _shorten_with_claude(text: str, max_chars: int) -> str:
             model="claude-sonnet-4-6",
             max_tokens=256,
             timeout=CLAUDE_TIMEOUT_S,
+            system=(
+                "You rewrite a message to fit within a strict character budget. "
+                "Output ONLY the rewritten message — no preamble, no quotes, no "
+                "explanation, no commentary, no labels, no character count. "
+                "Preserve the original meaning, tone, and voice. Plain text only."
+            ),
             messages=[
                 {
                     "role": "user",
-                    "content": f"Shorten this to {max_chars} characters: {text}",
+                    "content": (
+                        f"Rewrite the following message so it is at most "
+                        f"{max_chars} characters. Reply with the rewritten "
+                        f"message only.\n\n{text}"
+                    ),
                 }
             ],
         )