chore: ignore output/ — generator artifacts, not source

Every `openkb skill new` writes to `<kb>/output/skills/<name>/`. When
the KB happens to be the openkb repo itself (e.g. dogfooding the skill
factory from the project root), the untracked artifacts show up as
`??` in git status forever.

Goes alongside raw/, wiki/, and .openkb/ in the "Knowledge base test
artifacts" section — same intent: generator outputs and user data,
never source.

Note: `skills/openkb/` (the canonical dogfooded skill checked into
this repo) is at the top-level `skills/` directory, separate from
`output/skills/`, so this ignore does not accidentally hide committed
content.

Author: KylinMountain

.gitignore

@@ -13,6 +13,7 @@ venv/
 raw/
 wiki/
 .openkb/
+output/
 
 # Local only
 docs/