feat(lanes): name the files behind a lane's uncommitted counts

GET /api/lanes/:id/git now returns `files`, the raw `git status --porcelain`
lines the dirty/untracked counts were already parsed from, capped at 20 so a
lane mid-`npm install` cannot ship thousands of paths through a polled
endpoint. The lane card hovers them as the tooltip on its
"N modified · N untracked" text, appending "… +N" when the cap bit.

No new subprocess and no new endpoint — statusCounts was counting those lines
and throwing them away.

server/__tests__/worktree.test.js

@@ -100,6 +100,8 @@ describe("worktree", () => {
     assert.equal(s.dirty, 1);
     assert.equal(s.untracked, 1); // node_modules is ignored, so it does not count
     assert.match(s.head, /^[0-9a-f]{7,40}$/);
+    // The same lines the counts came from, so a caller can name the files.
+    assert.deepEqual(s.files.sort(), [" M README.md", "?? scratch.txt"]);
     // The fixture has no remotes. Given the lane's base branch, unpushedCount
     // measures base..HEAD — this worktree has committed nothing of its own, so 0.
     assert.equal(await wt.unpushedCount(dir, "main"), 0);