enhance(#3028): responsive Markdown separators instead of fixed-width rules (#3789)

* feat(#3028): responsive Markdown separators instead of fixed-width rules

Stage banners, checkpoints, completion and error panels used fixed-width
runs of box-drawing characters -- a 53-column heavy rule and a 62-column
double-line box. Those runs are ordinary text to a Markdown-rendering
host, so in a narrower pane they wrap and the border comes apart from
the heading it framed.

Shipped content now emits an ATX heading for a titled section and a
blank-line-delimited --- for a break between sections, both of which
adapt to the available width. The same convention is applied to the
three code sites that built these strings at runtime: the UAT
checkpoint renderer, the milestone-close audit report, and the TDD
review checkpoint table.

Removing the box also removes its only reason to exist -- the
east-asian-width padding helpers that kept its right border aligned
(checkpointBoxLine, displayWidth, isWideCodePoint, ZERO_WIDTH_MARK_RE,
CHECKPOINT_BOX_WIDTH). RTL directional isolation is unchanged.

The convention is specified in gsd-core/references/ui-brand.md and
enforced across all shipped content by tests/responsive-separators.test.cjs.

Refs #3028

* test(#3028): pin the heading form in checkpoint and audit-report assertions

These suites asserted the exact box borders and the 62-column padded
banner interior. With the box gone they assert the ### heading form,
the --- break and the bolded instruction line, and each now carries a
positive assertion that no box character remains -- which is what pins
the fix rather than merely tolerating it.

Language coverage is converted, not dropped: Japanese, Chinese, Korean,
Hindi and Arabic all still assert their rendered banner, and the Arabic
case still asserts the RTL directional isolates the box removal must
not disturb. Adds a case for a banner longer than the old inner width,
which previously produced a ragged border and now has none.

Refs #3028

* chore(#3028): acknowledge execute-plan.md growth from the checkpoint display spec

The checkpoint_protocol display spec described the drawn box; it now
describes the heading, the --- break and the bolded action prompt,
which costs 22 bytes (40111 -> 40133, 827 under the cap).

Appended to the existing #3370 fragment rather than filed as a new one:
a growth ack keys on the bare filename and #3370 already declares
execute-plan.md, so a second source naming it would be a hard
duplicate-key error. Same supersede-by-append route #3370 took for the
spent #2652 fragment.

Refs #3028

* docs(#3028): state the load-bearing half of the separator rule, and amend the zh-CN reference

Review found three things.

The rule as first written demanded a blank line above AND below every
---. Only the one above is load-bearing: it is what stops CommonMark
reading the rule as a setext underline for the line above. The one below
is cosmetic, because a thematic break is a leaf block. The rule now says
that, with the reason, instead of asserting a stricter form the content
does not keep.

The zh-CN reference had received the mechanical box-to-heading swap but
none of the prose behind it: it still claimed a 62-character checkpoint
width and still listed --- among forbidden mixed banner styles, so it
contradicted the convention it was translating. It now carries the
separator section, the setext reasoning, the unconditional-vs-per-runtime
rationale and a corrected anti-pattern list, in Chinese.

The user guide asserted that a heading is not a degradation anywhere.
That is an assertion, not a demonstration. It now says what was actually
traded away in a plain terminal, points at the recorded rationale, and
invites the report that would justify the capability flag instead.

Refs #3028

* chore(#3028): backfill changeset PR number

Refs #3028

---------

Co-authored-by: sim <sim@local>

src/audit.cts

@@ -1330,11 +1330,8 @@ function auditOpenArtifacts(cwd: string): AuditResult {
 function formatAuditReport(auditResult: AuditResult): string {
   const { counts, items, has_open_items, acknowledged } = auditResult;
   const lines: string[] = [];
-  const hr = '━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━';
 
-  lines.push(hr);
-  lines.push('  Milestone Close: Open Artifact Audit');
-  lines.push(hr);
+  lines.push('### Milestone Close: Open Artifact Audit');
 
   // WARNING 3 (#3458 follow-up review): the acknowledged tally previously
   // existed only in `--json` output — the human report could not tell
@@ -1343,12 +1340,12 @@ function formatAuditReport(auditResult: AuditResult): string {
   if (!has_open_items) {
     lines.push('');
     if (acknowledged.total > 0) {
-      lines.push(`  All artifact types clear (${acknowledged.total} previously acknowledged item${acknowledged.total !== 1 ? 's' : ''} still suppressed).`);
+      lines.push(`All artifact types clear (${acknowledged.total} previously acknowledged item${acknowledged.total !== 1 ? 's' : ''} still suppressed).`);
     } else {
-      lines.push('  All artifact types clear. Safe to proceed.');
+      lines.push('All artifact types clear. Safe to proceed.');
     }
     lines.push('');
-    lines.push(hr);
+    lines.push('---');
     return lines.join('\n');
   }
 
@@ -1459,12 +1456,12 @@ function formatAuditReport(auditResult: AuditResult): string {
   }
 
   lines.push('');
-  lines.push(hr);
-  lines.push(`  ${counts.total} item${counts.total !== 1 ? 's' : ''} require decisions before close.`);
+  lines.push('---');
+  lines.push('');
+  lines.push(`**${counts.total} item${counts.total !== 1 ? 's' : ''} require decisions before close.**`);
   if (acknowledged.total > 0) {
-    lines.push(`  ${acknowledged.total} previously acknowledged item${acknowledged.total !== 1 ? 's' : ''} also suppressed above the ${counts.total} open item${counts.total !== 1 ? 's' : ''}.`);
+    lines.push(`${acknowledged.total} previously acknowledged item${acknowledged.total !== 1 ? 's' : ''} also suppressed above the ${counts.total} open item${counts.total !== 1 ? 's' : ''}.`);
   }
-  lines.push(hr);
 
   return lines.join('\n');
 }