fix(story-mode): appends without invented line numbers, printed files you can open, whose words start a turn

The rest of review round 5 (a phone with touch):

- Text appended with `cat >> f <<EOF` or `echo x >> f` was drawn as lines
  1 to 8 of the file; where an append lands is not in the command, so the
  diff now says "Added at the end of the file (8 lines)" with no numbers.
- A command that prints a file and does something else (`cat textstats.py
  && pytest`) kept no record of the file; it is now on the event,
  resolved against the command's own folder, and the output drawer offers
  "Open textstats.py". (A file rewritten by a python here-document still
  cannot be diffed from the command alone; Claude Code's own "modified"
  note, when it comes, is used.)
- A background job's report that starts a turn was a balloon credited to
  "you" in the API (the page already drew it as a note); its speaker is
  now "note".
- The output drawer tagged a command still running in the background
  "ok"; it says "in the background".
- The Header buttons hint says that on a phone, Story Mode opens from
  Session Options.

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

test/story-mode-ui.test.ts

@@ -535,6 +535,34 @@ describe('story-mode.js overlay', () => {
     expect($$('.story-ln--add').map((l) => l.textContent)).toEqual(['1+const a = 1;']);
   });
 
+  it('an append draws without line numbers, and a check offers the file it printed', async () => {
+    const lines = [
+      prompt('add and test'),
+      ...bash(
+        'b1',
+        "cat >> notes.md <<'EOF'\nmore\nEOF\ncat stats.py && pytest -q",
+        'Append, show and test',
+        'def f(): pass\n5 passed in 0.1s'
+      ),
+      end(1000),
+    ];
+    (lines[1] as { cwd?: string }).cwd = '/work/demo-shop';
+    boot(storyOf(lines));
+    app.openStoryMode(SID);
+    await settle();
+    sizeAndDraw();
+    $('.story-panel[data-kind="check"] .story-hit')!.click();
+    const open = $$('.story-drawer-body .story-out-files button');
+    expect(open.map((b) => b.textContent)).toEqual(['Open stats.py']);
+    open[0].click();
+    expect(app.openFilePreview).toHaveBeenLastCalledWith('/work/demo-shop/stats.py', SID);
+    $$('.story-drawer-body button')
+      .find((b) => b.textContent === 'See the file it wrote')!
+      .click();
+    expect($('.story-ln--hunk')!.textContent).toBe('Added at the end of the file (1 line)');
+    expect($$('.story-ln--add').map((l) => l.textContent)).toEqual(['+more']);
+  });
+
   it('while a question waits, the pencils say so; the chosen answer is marked, not struck', async () => {
     const ask = (answered: boolean) =>
       storyOf([

test/story-mode.test.ts

@@ -975,6 +975,46 @@ describe('buildStoryPage', () => {
     expect(heredocWrites('tee -a notes.md <<EOF\nx\nEOF', '/w').map((d) => d.file)).toEqual(['/w/notes.md']);
   });
 
+  it('an append has no line numbers it cannot know; a whole-file write starts at line 1', () => {
+    const [append] = heredocWrites("cat >> notes.md <<'EOF'\none\ntwo\nEOF", '/w');
+    expect(append.hunks[0]).toMatchObject({ atEnd: true, newStart: 0, newLines: 2 });
+    const [echoed] = heredocWrites('echo "more" >> notes.md', '/w');
+    expect(echoed.hunks[0]).toMatchObject({ atEnd: true, newStart: 0 });
+    const [whole] = heredocWrites("cat > notes.md <<'EOF'\none\nEOF", '/w');
+    expect(whole.hunks[0]).toMatchObject({ newStart: 1, newLines: 1 });
+    expect(whole.hunks[0].atEnd).toBeUndefined();
+  });
+
+  it('files a command prints are on its event, resolved against its cwd, whatever it does', () => {
+    const lines = [
+      prompt('rewrite and test'),
+      ...bash('b1', 'cat textstats.py && python3 -m pytest -q', 'Show and test', 'def f(): pass\n5 passed in 0.1s'),
+      end(1000),
+    ];
+    (lines[1] as { cwd?: string }).cwd = '/w';
+    const page = pageOf(lines);
+    expect(page.events[1]).toMatchObject({ intent: 'test', files: ['/w/textstats.py'] });
+  });
+
+  it('words that start a turn but are not yours are a note, not your balloon', () => {
+    const turns = buildStoryTurns(
+      entriesOf([
+        prompt('go'),
+        end(1000),
+        prompt(
+          '<task-notification>\n<task-id>bg1</task-id>\n<status>completed</status>\n<summary>Background command \"Run tests\" completed (exit code 0)</summary>\n</task-notification>',
+          { turnOrigin: 'task_notification' }
+        ),
+        said('All good.'),
+        end(1000),
+      ])
+    );
+    expect(buildStoryPage(turns[0]).panels[0].balloon?.who).toBe('you');
+    const report = buildStoryPage(turns[1]).panels[0];
+    expect(report.caption.story).toBe('A background job reports back.');
+    expect(report.balloon?.who).toBe('note');
+  });
+
   it('a loop keeps every run: a lint failure inside a round that passed in the end still shows', () => {
     const page = pageOf([
       prompt('build'),