Ignore the local outputs/ folder

Scratch exports from running the app and the CLI land in outputs/. It is
not ignored, so `cargo xtask assets` — which scans tracked and untracked
files alike — reports every PNG and PDF in it as an asset with no row in
ASSETS.md, and the check cannot pass on a machine that has ever run the
app with an output directory.

.gitignore

@@ -4,6 +4,7 @@ plan/
 /dist
 /corpus
 /log
+/outputs/
 .DS_Store
 /.claude/
 /target-*/