[3.15] gitignore a do-not-commit/ directory (GH-160199) (#160494)

a non-branded scratch dir for stuff we or our agents do in our worktrees.

(cherry picked from commit 3a3d39c)

Author: gpshead

.gitignore

@@ -181,3 +181,6 @@ Python/frozen_modules/MANIFEST
 # People's custom https://docs.anthropic.com/en/docs/claude-code/memory configs.
 /.claude/
 CLAUDE.local.md
+
+# Local scratch space for things we or our agents do in our worktrees.
+/do-not-commit/