feat(observer): strip <system-reminder> blocks from promptText

Closes brain-retro 2026-05-20 #8 — UserPromptSubmit hook injects
<system-reminder>...</system-reminder> blocks into user.content that
polluted classifyTask / classifyPromptSignal / routing detection.
Now stripped via regex before any analysis.

Completed by controller (Opus) after subagent hit context limit on
1250-line test file. Helper stripSystemReminders + promptText update
were committed by subagent; test cases appended via Bash heredoc.

4 new vitest tests, 290/290 GREEN.

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

tools/observer-transcript-parser.mjs

@@ -95,14 +95,19 @@ function findTurnStart(entries) {
   return 0;
 }
 
+function stripSystemReminders(text) {
+  return String(text || '').replace(/<system-reminder>[\s\S]*?<\/system-reminder>/g, '');
+}
+
 function promptText(entry) {
   const c = entry && entry.message && entry.message.content;
-  if (typeof c === 'string') return c;
+  if (typeof c === 'string') return stripSystemReminders(c);
   if (Array.isArray(c)) {
-    return c
+    const joined = c
       .filter((b) => b && b.type === 'text')
       .map((b) => b.text || '')
       .join(' ');
+    return stripSystemReminders(joined);
   }
   return '';
 }