fix: strip <system-reminder> tags from persisted memory and DRY up regex

System reminders (CLAUDE.md contents, deferred tool lists) were being
stored in memory observations. Add system-reminder to the tag stripping
pipeline alongside <private> and <system_instruction>, and extract the
duplicated regex into a shared SYSTEM_REMINDER_REGEX constant.

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

src/utils/transcript-parser.ts

@@ -5,6 +5,7 @@
 
 import { readFileSync } from 'fs';
 import { logger } from './logger.js';
+import { SYSTEM_REMINDER_REGEX } from './tag-stripping.js';
 import type {
   TranscriptEntry,
   UserTranscriptEntry,
@@ -163,7 +164,7 @@ export class TranscriptParser {
 
       if (filterSystemReminders) {
         // Filter out system-reminder tags and their content
-        text = text.replace(/<system-reminder>[\s\S]*?<\/system-reminder>/g, '');
+        text = text.replace(SYSTEM_REMINDER_REGEX, '');
         // Clean up excessive whitespace
         text = text.replace(/\n{3,}/g, '\n\n').trim();
       }