fix(server): name the real cost of indexing untracked files

Measured on 50k untracked files: git add --pathspec-from-file took 26 of
29.5 seconds. Also guard the test against an empty source list.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: t3dotgg

apps/server/src/vcs/GitVcsDriverCore.test.ts

@@ -2176,6 +2176,7 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
         assert.isUndefined(status.branchChanges);
 
         const preview = yield* driver.getReviewDiffPreview({ cwd });
+        assert.isNotEmpty(preview.sources);
         assert.isTrue(preview.sources.every((source) => source.truncated));
       }),
     );

apps/server/src/vcs/GitVcsDriverCore.ts

@@ -59,8 +59,9 @@ const RANGE_DIFF_SUMMARY_MAX_OUTPUT_BYTES = 19_000;
 const RANGE_DIFF_PATCH_MAX_OUTPUT_BYTES = 59_000;
 const REVIEW_DIFF_PATCH_MAX_OUTPUT_BYTES = 120_000;
 const REVIEW_METADATA_MAX_OUTPUT_BYTES = 16 * 1024 * 1024;
-// Counting untracked files copies the index and stats every one of them on each status refresh.
-// Past this many, the cost is a CPU spike per agent turn, so the totals are reported incomplete.
+// Every status refresh adds the untracked files to a temporary index, and git add's cost grows
+// much faster than the file count (50k files took about 30 seconds). Past this many, the
+// totals are reported incomplete instead.
 const REVIEW_UNTRACKED_MAX_FILES = 5_000;
 const REVIEW_DIFF_FILE_MAX_OUTPUT_BYTES = 1024 * 1024;
 // Patches the clients render are parsed against git's default a/ and b/ path