Ignore the `gish` `github/` local issue cache

`gh issue create` caches bodies under the
`<backend>/<repo>/<kind>/<num>.md` path — and that dir is named
for the *service*, not the CLI, so the existing `gitea/` + `gh/`
entries never covered it. Filing the `tipc` follow-ups (#495-501)
is what surfaced it.

(this patch was generated in some part by `claude-code` using `claude-opus-5` (`anthropic`))

.gitignore

@@ -161,6 +161,10 @@ Session.vim
 # -[ ] everything should be put under a `.gish/` no?
 gitea/
 gh/
+# NOTE, `gh issue create` writes its local cache under the
+# `<backend>/<repo>/<kind>/<num>.md` path, so the dir is named for
+# the *service* not the CLI.
+github/
 
 # ------ macOS ------
 # Finder metadata