Add a summary/stats line to the worklog #4

What

At export time, prepend a one-line "at a glance" summary to the worklog with aggregate counts for the selected window.

Suggested contents

• commits
• PRs authored
• PRs reviewed
• repos touched
• net lines changed (+/-), derived from the git show --stat data already fetched for the preview pane

Example:

8 commits · 3 PRs · 2 reviews · 4 repos · +1,204 / -318

Notes

• Should respect the current selection (only selected items count), and the chosen format (md / json). For JSON, emit a structured summary object rather than a string.
• Line-change numbers are already available via git show --stat (see internal/collect/git.go Preview); may need a lightweight numstat fetch to total them without the diff text.

Relates to the feature brainstorm in chat (#1).