fix(ci): Preserve leading prose indentation in ownDescription

canonicalizeBody derives the description via ownDescription, which used
.trim() — stripping leading indentation from a managed PR's prose, so a
description opening with an indented Markdown code block could be silently
reflowed. Tidy only the whitespace the region surgery introduces: collapse
seams, drop leading blank lines, and trim trailing whitespace, leaving the
first line's indentation intact.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: theCodeDrift

.github/scripts/stack-breadcrumb.cjs

@@ -201,15 +201,23 @@ function extractCarriedRegions(body) {
 
 /**
  * A body's own description — the human-written part, with the managed regions
- * (the stack tree and any carried `<!-- PR:N -->` regions) removed and spacing
- * tidied.
+ * (the stack tree and any carried `<!-- PR:N -->` regions) removed and the
+ * spacing their removal left behind tidied.
+ *
+ * Only whitespace introduced by the surgery is touched: 3+ newline runs (a seam
+ * where a region was cut from the middle) collapse to a blank line, leading
+ * blank lines (left when a region sat at the very top) are dropped, and trailing
+ * whitespace is trimmed. The prose is otherwise preserved verbatim — crucially,
+ * leading indentation on the first line survives, so a description that opens
+ * with an indented Markdown code block is not silently reflowed.
  */
 function ownDescription(body) {
   return body
     .replace(REGION_PATTERN, "")
     .replace(ANY_PR_REGION_PATTERN, "")
     .replace(/\n{3,}/g, "\n\n")
-    .trim();
+    .replace(/^\n+/, "")
+    .trimEnd();
 }
 
 /** Wrap a description as a carried region for `number_`. */

.github/scripts/stack-breadcrumb.test.cjs

@@ -581,6 +581,14 @@ test("ownDescription: strips the stack tree and carried regions", () => {
   assert.equal(ownDescription(body), "My description.");
 });
 
+test("ownDescription: preserves leading indentation after a top region", () => {
+  // A breadcrumb at the very top must not cost the prose its leading indent —
+  // otherwise a description opening with an indented code block gets reflowed.
+  const body =
+    "<!-- stack root=1 pr=1,2:1 -->\n(tree)\n<!-- /stack -->\n\n    const x = 1;\nplain line";
+  assert.equal(ownDescription(body), "    const x = 1;\nplain line");
+});
+
 test("upsertCarriedRegion: appends when absent, replaces in place when present", () => {
   const appended = upsertCarriedRegion("Body.", 85, P85);
   assert.equal(appended, `Body.\n\n${P85}`);