The repository's own .github, now that it is public

SPEC 2.2a says INBUXA writes its own when the repository is first published,
and it is. Until now the public repository carried Stalwart's: a security
policy telling people to report vulnerabilities to Stalwart Labs, and a
contributing guide whose policy is that pull requests from anyone not on
upstream's vouched list are closed automatically. Neither is this project's,
and both were being offered to anyone who looked.

So: a security policy that says where to send a report, and what happens if
it turns out to be upstream's bug rather than ours; a contributing guide that
says what a fork of someone else's code needs from a contributor, including
the clean-room question, since the record has to stay true; the Contributor
Covenant; and a sponsor link. Upstream's two security documents move to
.github-upstream/ beside its workflows -- kept, not used, not presented as
ours.

CI builds the server and compiles every test target, and deliberately runs
no suite. The unit tests only build with the integration crate in the graph,
and the integration suites want a STORE, fixed ports and a container apiece,
so running them here would mean a tick that skipped everything or a cross
that means "the runner has no Redis". The workflow says as much, so nobody
has to rediscover it.

Also ignores /artifact: two hand-built binaries, ~190 MB, one `git add -A`
away from a public repository.

.gitignore

@@ -1,4 +1,7 @@
 /target
+# Release binaries built by hand: ~100 MB each, and a public repository is
+# the wrong place for them.
+/artifact
 *.failed
 *_failed
 run.sh