feat(app): add friendly context message to bug report prompt (#1959)

## Motivation

When a user clicks **Send Bug Report** in the macOS app, we already give
them the option to add more context via an optional text field. But the
current prompt is just a terse label — `"What's the issue? (optional)"`
— which doesn't tell the user why bothering to fill it in matters. A
friendly one-line explanation increases the chance they'll describe what
went wrong, which is the single most useful signal when we triage the
resulting diagnostic bundle.

## Changes

- `app/EXO/EXO/ContentView.swift`: In the `.prompting` phase of
`sendBugReportButton`, replace the single label with a two-line
hierarchy:
  - Primary: `Tell us what went wrong (optional)`
- Helper: `A quick description of what you were doing and what happened
helps us track down the bug for you.`
- The helper uses `.caption2` + `.secondary` + `.opacity(0.8)` +
`.fixedSize(horizontal: false, vertical: true)` so it stays visually
subordinate and wraps cleanly inside the 340pt popover.

No changes to `BugReportService`, the `user_description` payload, or any
other flow.

## Why It Works

The optional description is already plumbed end-to-end (text editor →
`bugReportUserDescription` state → `BugReportService.sendReport(...,
userDescription:)` → `report.json`'s `user_description` field → GitHub
issue pre-fill). The only gap was user-facing motivation, so this is
purely a copy/layout tweak inside the existing `.prompting` case — no
new state, bindings, or service changes.

## Test Plan

### Manual Testing
<!-- Hardware: MacBook Pro (macOS app build via Xcode) -->
- Build the macOS app in Xcode (`app/EXO/EXO.xcodeproj`) and launch it.
- Open the menubar popover → expand **Debug Info** → click **Send Bug
Report**.
- Verify the new primary label and helper sentence both appear above the
text editor and wrap cleanly within the popover width.
- Leave the field empty → click **Send** → upload should succeed (no
`user_description` in payload, same as before).
- Fill in a description → click **Send** → upload succeeds and the
success card with **Create GitHub Issue** appears; clicking it opens
GitHub with the description pre-filled.
- Click **Cancel** from the prompting state → returns to idle.

### Automated Testing
<!-- Describe changes to automated tests, or how existing tests cover
this change -->
- No new automated tests. This is a SwiftUI copy/layout change; existing
`EXOTests` are smoke-level and don't cover `ContentView` view bodies,
and UI snapshot tests aren't worth adding for a two-line copy tweak.
- `nix fmt` reports 0 files changed after the edit; `nix flake check` in
CI will verify formatting/lint for the rest of the tree.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/EXO/EXO/ContentView.swift

@@ -584,9 +584,18 @@ struct ContentView: View {
 
             case .prompting:
                 VStack(alignment: .leading, spacing: 6) {
-                    Text("What's the issue? (optional)")
+                    VStack(alignment: .leading, spacing: 2) {
+                        Text("Tell us what went wrong (optional)")
+                            .font(.caption2)
+                            .foregroundColor(.secondary)
+                        Text(
+                            "A quick description of what you were doing and what happened helps us track down the bug for you."
+                        )
                         .font(.caption2)
                         .foregroundColor(.secondary)
+                        .opacity(0.8)
+                        .fixedSize(horizontal: false, vertical: true)
+                    }
                     TextEditor(text: $bugReportUserDescription)
                         .font(.caption2)
                         .frame(height: 60)