Escape Markdown in alert signals and cap OSINT text in ideas prompt

Addresses PR review: escape Markdown-sensitive characters in
_formatTieredAlert signal bullets to prevent Telegram Bot API
rejections, and add a 1500-char budget for URGENT_OSINT in
compactSweepForLLM to bound prompt size while keeping full text upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/alerts/telegram.mjs

@@ -730,7 +730,7 @@ Respond with ONLY valid JSON:
     if (evaluation.signals?.length) {
       lines.push('', `*Signals:*`);
       for (const sig of evaluation.signals) {
-        lines.push(`• ${sig}`);
+        lines.push(`• ${escapeMd(sig)}`);
       }
     }
 
@@ -742,6 +742,11 @@ Respond with ONLY valid JSON:
 
 // ─── Helpers ──────────────────────────────────────────────────────────────
 
+function escapeMd(text) {
+  if (!text) return '';
+  return text.replace(/([_*\[\]()~`>#+\-=|{}.!\\])/g, '\\$1');
+}
+
 function parseJSON(text) {
   if (!text) return null;
   let cleaned = text.trim();