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/services/context/ObservationCompiler.ts

@@ -8,6 +8,7 @@ import path from 'path';
 import { existsSync, readFileSync } from 'fs';
 import { SessionStore } from '../sqlite/SessionStore.js';
 import { logger } from '../../utils/logger.js';
+import { SYSTEM_REMINDER_REGEX } from '../../utils/tag-stripping.js';
 import { CLAUDE_CONFIG_DIR } from '../../shared/paths.js';
 import type {
   ContextConfig,
@@ -164,7 +165,7 @@ export function extractPriorMessages(transcriptPath: string): PriorMessages {
               text += block.text;
             }
           }
-          text = text.replace(/<system-reminder>[\s\S]*?<\/system-reminder>/g, '').trim();
+          text = text.replace(SYSTEM_REMINDER_REGEX, '').trim();
           if (text) {
             lastAssistantMessage = text;
             break;