fix(inbox): break signal summary sections onto separate lines

[MattPua]

Problem

Signals had sections in the same line, which means it can be difficult to skim through quickly.

Changes

Insert line breaks around What's happening, Root cause, and How to resolve headers so report summaries are easier to scan.

How did you test this?

Locally.

Publish to changelog?

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
apps/code/src/renderer/features/inbox/components/utils/SignalReportSummaryMarkdown.tsx:27-32
The `listMarkdown` computation splits `raw` on the first newline, but after formatting, `raw` now begins with `**What's happening:**

…`, so `listMarkdown` becomes just `"**What's happening:**"`. The `list` variant then renders only the bold header (e.g. "What's happening:") with no actual summary content — a regression from the previous behaviour where the full inline text was visible. Using the pre-formatted content for the list split preserves the original single-line text for the list preview.

```suggestion
  const rawContent = content?.trim() ? content : fallback;
  const raw = formatSignalReportSummaryMarkdown(rawContent);

  /** List rows: only the first line (before first newline); CSS still caps visual lines. */
  const listMarkdown = rawContent.split(/\r?\n/)[0] ?? "";
```

### Issue 2 of 2
apps/code/src/renderer/features/inbox/utils/formatSignalReportSummaryMarkdown.test.ts:4-33
All four cases share the same `(input, expected)` shape and should be collapsed into a single `it.each` table, per the team's preference for parameterised tests.

_{Reviews (1): Last reviewed commit: "fix(inbox): break signal summary section..." | Re-trigger Greptile}