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>

tests/audit-command-cutover.test.cjs

@@ -238,8 +238,8 @@ describe('audit cutover: output shape equivalence', () => {
       `audit-open must succeed. stderr: ${result.error}`);
     const lines = result.output.split('\n').map(l => l.trim()).filter(Boolean);
     assert.ok(
-      lines.includes('Milestone Close: Open Artifact Audit'),
-      `report title must appear as a standalone line; got: ${JSON.stringify(lines.slice(0, 5))}`,
+      lines.includes('### Milestone Close: Open Artifact Audit'),
+      `report title must appear as a standalone Markdown heading line; got: ${JSON.stringify(lines.slice(0, 5))}`,
     );
   });
 
@@ -481,7 +481,7 @@ describe('audit-open — does not crash with ReferenceError (#2659)', () => {
  *   2. Even after switching to `core.output(formatted, raw)`, the human-readable
  *      branch JSON-stringifies the formatted string because `core.output` only
  *      bypasses JSON encoding when called as `core.output(null, true, rawValue)`.
- *      Result: stdout contains `"━━━…\n  Milestone Close: …\n…"` (a JSON string
+ *      Result: stdout contains `"### Milestone Close: …\n…"` (a JSON string
  *      literal) instead of the rendered report.
  *
  * The shape assertions below catch both regressions structurally — never via
@@ -640,10 +640,10 @@ describe('audit-open — output shape (#2911)', () => {
 
     const lines = result.output.split('\n').map(l => l.trim()).filter(Boolean);
 
-    // The first non-empty line must be the divider character row, *not* a
-    // JSON-encoded string starting with a quote. If core.output JSON-stringified
+    // The first non-empty line must be the rendered Markdown heading row, *not*
+    // a JSON-encoded string starting with a quote. If core.output JSON-stringified
     // the formatted report, the entire payload sits on one line wrapped in
-    // double quotes ("━━━…\n…").
+    // double quotes ("### Milestone Close: …\n…").
     assert.ok(
       !result.output.startsWith('"'),
       'text-mode stdout must not begin with a JSON quote (would mean the report was JSON.stringified)'
@@ -655,8 +655,8 @@ describe('audit-open — output shape (#2911)', () => {
 
     // Section headers from formatAuditReport that must appear as standalone lines.
     assert.ok(
-      lines.includes('Milestone Close: Open Artifact Audit'),
-      `expected report title as a standalone line; got lines: ${JSON.stringify(lines.slice(0, 5))}`
+      lines.includes('### Milestone Close: Open Artifact Audit'),
+      `expected report title as a standalone Markdown heading line; got lines: ${JSON.stringify(lines.slice(0, 5))}`
     );
     assert.ok(
       lines.includes('All artifact types clear. Safe to proceed.'),