Show a lazy git dirty-count on the highlighted New-screen row

[roypadina]

What

New-screen rows already show the git branch (src/core/git.ts readGitBranch, zero-spawn). It'd be nice to also show an uncommitted-changes indicator (e.g. ±3) — but only for the currently highlighted row, so we never shell out to git for every row on the hot scan path.

Why lazy / highlighted-only

CLAUDE.md calls this out explicitly: don't spawn git per row on the scan path; "a future lazy dirty-count for the highlighted row only is the sole allowed exception." So this is the sanctioned way to add it.

Pointers

• src/core/git.ts — add a small countDirty(dir): Promise<number> that runs git status --porcelain once for a single dir (this is the one allowed spawn).
• src/cli/screens/NewScreen.tsx — fetch the count for the highlighted dir only (debounced on cursor move), render it next to the branch (e.g. ⎇ main ±3).
• Add a unit test for parsing git status --porcelain output → count.

Acceptance

• Highlighted row shows ±N when the repo has uncommitted changes; nothing for clean repos.
• No extra git spawns for non-highlighted rows.
• Test covers the porcelain-count parser.