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>

gsd-core/workflows/thread.md

@@ -42,12 +42,14 @@ Apply filter for LIST-OPEN (show only status=open or status=in_progress) or LIST
 Display:
 ```
 Context Threads
-─────────────────────────────────────────────────────────
+
+---
 slug                      status        updated      title
 auth-decision             open          2026-04-09   OAuth vs Session tokens
 db-schema-v2              in_progress   2026-04-07   Connection pool sizing
 frontend-build-tools      resolved      2026-04-01   Vite vs webpack
-─────────────────────────────────────────────────────────
+
+---
 3 threads (2 open/in_progress, 1 resolved)
 ```
 
@@ -96,7 +98,8 @@ When SUBCMD=status and SLUG is set (already sanitized):
 2. Read the file and display a summary:
    ```
    Thread: {SLUG}
-   ─────────────────────────────────────
+
+---
    Title:   {title from frontmatter or # heading}
    Status:  {status from frontmatter or ## Status heading}
    Updated: {updated from frontmatter}
@@ -107,7 +110,8 @@ When SUBCMD=status and SLUG is set (already sanitized):
 
    Next Steps:
    {content of ## Next Steps section}
-   ─────────────────────────────────────
+
+---
    Resume with: /gsd:thread {SLUG}
    Close with:  /gsd:thread close {SLUG}
    ```