refactor(display): remove details prepend path, add Matrix reasoning via formatReasoningDisplay

- Remove formatReasoningAsCodeBlock (was experimental, leaked into PR)
- Remove htmlPrefix from OutboundMessage and editMessage signatures
- Add Matrix-specific <details> collapsible to formatReasoningDisplay
- Matrix adapter handles parseMode='HTML' to skip double-escaping
- Reasoning now flows through the standard display path like all adapters

src/channels/types.ts

@@ -21,7 +21,7 @@ export interface ChannelAdapter {
   
   // Messaging
   sendMessage(msg: OutboundMessage): Promise<{ messageId: string }>;
-  editMessage(chatId: string, messageId: string, text: string, htmlPrefix?: string): Promise<void>;
+  editMessage(chatId: string, messageId: string, text: string): Promise<void>;
   sendTypingIndicator(chatId: string): Promise<void>;
   stopTypingIndicator?(chatId: string): Promise<void>;
 

src/core/display.ts

@@ -230,6 +230,19 @@ export function formatReasoningDisplay(
     // Signal: no blockquote support, use italic
     return { text: `**Thinking**\n_${truncated}_` };
   }
+  if (channelId === 'matrix') {
+    // Matrix: collapsible <details> block — rendered natively by Element Web,
+    // falls back to visible block on mobile clients.
+    const escaped = truncated
+      .replace(/&/g, '&amp;')
+      .replace(/</g, '&lt;')
+      .replace(/>/g, '&gt;')
+      .replace(/\n/g, '<br>');
+    return {
+      text: `<details><summary>🧠 Thinking</summary>${escaped}</details>`,
+      parseMode: 'HTML',
+    };
+  }
   if (channelId === 'telegram' || channelId === 'telegram-mtproto') {
     // Telegram: use HTML blockquote to bypass telegramify-markdown spacing.
     // Convert basic markdown inline formatting to HTML tags so bold/italic
@@ -255,36 +268,6 @@ export function formatReasoningDisplay(
   return { text: `> **Thinking**\n${quoted}` };
 }
 
-/**
- * Format reasoning as a collapsible <details> block for prepending to the response.
- * Returns pre-escaped HTML meant to be injected directly into formatted_body
- * (bypasses the adapter's markdown-to-HTML conversion to avoid double-escaping).
- */
-export function formatReasoningAsCodeBlock(
-  text: string,
-  channelId?: string,
-  reasoningMaxChars?: number,
-): { text: string } | null {
-  const maxChars = reasoningMaxChars ?? 0;
-  const cleaned = text.split('\n').map(line => line.trimStart()).join('\n').trim();
-  if (!cleaned) return null;
-
-  const truncated = maxChars > 0 && cleaned.length > maxChars
-    ? cleaned.slice(0, maxChars) + '...'
-    : cleaned;
-
-  // HTML-escape the reasoning content, then convert newlines to <br>
-  const escaped = truncated
-    .replace(/&/g, '&amp;')
-    .replace(/</g, '&lt;')
-    .replace(/>/g, '&gt;')
-    .replace(/\n/g, '<br>');
-
-  return {
-    text: `<details><summary>🧠 Thinking</summary><br>${escaped}</details><br>`,
-  };
-}
-
 /**
  * Format AskUserQuestion options for channel display.
  */

src/core/types.ts

@@ -132,9 +132,6 @@ export interface OutboundMessage {
    *  'HTML') and to skip its default markdown conversion. Adapters that don't
    *  support the specified mode ignore this and fall back to default. */
   parseMode?: string;
-  /** Pre-escaped HTML to prepend to formatted_body only (bypasses markdown conversion).
-   *  Used for reasoning blocks with <details> tags that would be double-escaped. */
-  htmlPrefix?: string;
 }
 
 /**