Extract the inline tree-building logic from IssuesList into a pure
`buildIssueTree` function in lib/issue-tree.ts so it can be unit tested.
Add six tests covering: flat lists, parent-child grouping, multi-level
nesting, orphaned sub-tasks promoted to root, empty input, and list
order preservation.
Add two tests to IssueRow.test.tsx covering the new titleSuffix prop:
renders inline after the title when provided, and renders cleanly when
omitted.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
