fix: change compaction output to HTML comment, prevent Markdown rendering issues

Root cause: Model was instructed to output <workspace_memory_candidates> XML
tags in the user-visible compaction summary, causing purple/italic rendering
when combined with --- delimiters in Markdown.

Fixes:
- compactionContextHeader(): Now instructs model to use HTML comment format
  <!-- workspace_memory_candidates ... --> which is hidden from users
- extractCandidateBlock(): New function supports 3 formats:
  1. HTML comment (preferred, hidden from user)
  2. Markdown section (visible but clean)
  3. Legacy XML (backward compatible)
- Added "DO NOT use XML tags" and "DO NOT start with ---" instructions

Tests:
- Verify compaction context header uses HTML comment format
- Test parser accepts all 3 formats (HTML comment, Markdown, legacy XML)

src/extractors.ts

@@ -221,14 +221,34 @@ function shouldAcceptWorkspaceMemoryCandidate(entry: {
   return true;
 }
 
+/**
+ * Extract candidate block from summary using multiple formats.
+ * Supports: HTML comment, Markdown section, legacy XML.
+ */
+function extractCandidateBlock(summary: string): string | null {
+  // 1. HTML comment block (preferred, hidden from user)
+  const commentMatch = summary.match(/<!--\s*workspace_memory_candidates\s*\n([\s\S]*?)-->/i);
+  if (commentMatch) return commentMatch[1];
+
+  // 2. Markdown section (visible but clean)
+  const markdownMatch = summary.match(/##\s*Workspace Memory Candidates\s*\n([\s\S]*?)(?:\n##|$)/i);
+  if (markdownMatch) return markdownMatch[1];
+
+  // 3. Legacy XML block (backward compatible)
+  const xmlMatch = summary.match(/<workspace_memory_candidates>([\s\S]*?)<\/workspace_memory_candidates>/i);
+  if (xmlMatch) return xmlMatch[1];
+
+  return null;
+}
+
 export function parseWorkspaceMemoryCandidates(summary: string): LongTermMemoryEntry[] {
-  const match = summary.match(/<workspace_memory_candidates>([\s\S]*?)<\/workspace_memory_candidates>/i);
-  if (!match) return [];
+  const block = extractCandidateBlock(summary);
+  if (!block) return [];
 
   const now = new Date().toISOString();
   const entries: LongTermMemoryEntry[] = [];
 
-  for (const line of match[1].split("\n")) {
+  for (const line of block.split("\n")) {
     const item = line.trim().match(/^-\s*\[(feedback|project|decision|reference)\]\s*(.+)$/i);
     if (!item) continue;
     const type = item[1].toLowerCase() as LongTermType;

src/plugin.ts

@@ -73,14 +73,16 @@ The following sections are PRIVATE INPUT for updating the compaction summary.
 DO NOT copy these sections, their headings, or their contents verbatim.
 Use the facts only to update the normal summary sections (Goal, Progress, etc.).
 
-At the VERY END of your summary, you MAY include ONE output block:
+At the VERY END of your summary, if there are durable memory candidates, include this hidden block:
 
-<workspace_memory_candidates>
+<!-- workspace_memory_candidates
 - [type] content (types: feedback, project, decision, reference)
-</workspace_memory_candidates>
+-->
 
 Only include truly durable information useful across FUTURE sessions.
 If nothing qualifies, omit the block entirely.
+DO NOT use XML tags like <workspace_memory_candidates>.
+DO NOT start with "---" frontmatter delimiters.
 
 [END PRIVATE COMPACTION CONTEXT]
 `.trim();