fix: use HTML comment format for dcp-id tags to prevent model echo

XML-style <dcp-id>m001</dcp-id> tags get echoed by models because they
look like semantic content. HTML comments <!-- dcp-id: m001 --> are
invisible in rendered markdown and models are trained to skip them.

prompts.ts

@@ -13,7 +13,7 @@ You operate in a context-constrained environment. Manage context continuously to
 
 The ONLY tool you have for context management is \`compress\`. It replaces older conversation content with technical summaries you produce.
 
-\`<dcp-id>\` and \`<dcp-system-reminder>\` tags are environment-injected metadata. Do not output them.
+\`<!-- dcp-id: ... -->\` and \`<dcp-system-reminder>\` tags are environment-injected metadata. Do not output them.
 
 THE PHILOSOPHY OF COMPRESS
 \`compress\` transforms conversation content into dense, high-fidelity summaries. This is not cleanup — it is crystallization. Your summary becomes the authoritative record of what transpired.
@@ -114,7 +114,7 @@ You specify boundaries by ID using the injected IDs visible in the conversation:
 - \`mNNN\` IDs identify raw messages (3 digits, zero-padded, e.g. \`m001\`, \`m042\`)
 - \`bN\` IDs identify previously compressed blocks
 
-Each message has an ID inside XML metadata tags like \`<dcp-id>...</dcp-id>\`.
+Each message has an ID inside an HTML comment like \`<!-- dcp-id: m001 -->\`.
 The ID tag appears at the end of the message it belongs to — it identifies the message above it, not the one below it.
 Treat these tags as boundary metadata only, not as tool result content.
 
@@ -215,7 +215,7 @@ Prefer multiple short, closed ranges over one large range when several independe
 export const MANUAL_MODE_SYSTEM_PROMPT = `
 You are operating in DCP manual mode for context management.
 
-\`<dcp-id>\` and \`<dcp-system-reminder>\` tags are environment-injected metadata. Do not output them.
+\`<!-- dcp-id: ... -->\` and \`<dcp-system-reminder>\` tags are environment-injected metadata. Do not output them.
 
 In manual mode you do NOT proactively compress conversation content. Compression is a deliberate, user-directed action.
 

pruner.ts

@@ -151,9 +151,9 @@ function applyCompressionBlocks(messages: any[], state: DcpState): any[] {
             block.topic +
             "]\n\n" +
             block.summary +
-            "\n\n<dcp-block-id>b" +
+            "\n\n<!-- dcp-block-id: b" +
             block.id +
-            "</dcp-block-id>",
+            " -->",
         },
       ],
       // anchorTimestamp is always finite (resolveAnchorTimestamp returns
@@ -356,11 +356,11 @@ function injectMessageIds(messages: any[], state: DcpState): void {
     const id = "m" + String(counter).padStart(3, "0");
     counter++;
 
-    const idTag = `\n<dcp-id>${id}</dcp-id>`;
+    const idTag = `\n<!-- dcp-id: ${id} -->`;
 
     if (role === "user") {
       if (typeof msg.content === "string") {
-        msg.content = msg.content + `\n\n<dcp-id>${id}</dcp-id>`;
+        msg.content = msg.content + `\n\n<!-- dcp-id: ${id} -->`;
       } else if (Array.isArray(msg.content)) {
         msg.content = [...msg.content, { type: "text", text: idTag }];
       }