fix(#592): a rule inside a Copilot answer stays part of it in agent read

The pane segmenter ended the response block at every full-width rule, so
an answer that draws one (a markdown `---`, a passage quoted between two
rules) read as only its last section: `Summary / ──── / Details` came
back as "Details", and the existing test pinned that by expecting the
quoted passage without its "Here it is:" lead-in.

Inside an open response a rule is now held and written into the text as
`---` once more of the same answer follows, so a rule that only separates
the answer from the footer never trails it. The composer's own two rules
(the last rule/❯/rule triple) are still boundaries. The old test now
expects both parts, and a new one pins a Summary/rule/Details answer as
one block; both fail without the change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/response-viewer-transcript.test.ts

@@ -551,10 +551,33 @@ describe('GitHub Copilot pane dialect', () => {
       ...tail,
     ].join('\n');
     expect(getLastTranscriptResponse(parseExternalCliTranscript(screen, 'copilot'))).toBe(
-      'Quoted passage between two rules.'
+      'Here it is:\n\n---\n\nQuoted passage between two rules.'
     );
   });
 
+  it('keeps an answer whole across a rule inside it, so agent read never returns only its tail', () => {
+    const screen = [
+      ' ❯ summarize the change   11:22',
+      ' ● Summary',
+      '',
+      '   One line of summary.',
+      rule,
+      '   Details',
+      '',
+      '   The longer explanation.',
+      ...tail,
+    ].join('\n');
+    const blocks = parseExternalCliTranscript(screen, 'copilot');
+    expect(blocks.filter((b) => b.kind === 'response')).toHaveLength(1);
+    const answer = getLastTranscriptResponse(blocks);
+    expect(answer).toContain('Summary');
+    expect(answer).toContain('One line of summary.');
+    expect(answer).toContain('Details');
+    expect(answer).toContain('The longer explanation.');
+    // The rule before the footer separates the answer from the chrome and never trails it.
+    expect(answer.endsWith('---')).toBe(false);
+  });
+
   it('answers nothing for a session that has only drawn its banner or a trust dialog', () => {
     const banner = pane.split('\n').slice(0, 9).join('\n');
     expect(getLastTranscriptResponse(parseExternalCliTranscript(banner, 'copilot'))).toBe('');