Drop the 18 MB simulator binary I committed, and stop it happening again

`go build ./...` inside a main package writes its binary beside the
source, and the fleet commit's `git add -A` took it: fa7c6c0 carries
hack/demo-simulator/demo-simulator at 18,756,186 bytes. Nothing needs it
in the repo -- the demo box gets a binary built for linux/amd64 and
pushed to /home/john/cairnobs-demo/bin/, not this one -- and it landed on
main before anybody noticed.

Removing it here stops the working tree carrying it and stops it growing
with every rebuild, but it does not remove it from history: the blob is
in fa7c6c0 for good unless main is rewritten, so every clone pays 18 MB
for a file no clone uses. Worth deciding deliberately rather than by
default; this change is the half that needs no decision.

The .gitignore entries name all five main packages under hack/ whose
directory name is also their binary name, not just the one that got
caught, since the same `go build ./...` in any of them produces the same
artefact and the next one would land the same way. Checked: none of the
other four is currently tracked, and no other binary over 1 MB is.

.gitignore

@@ -31,3 +31,13 @@ docker-compose.override.yml
 .DS_Store
 Thumbs.db
 *.swp
+
+# `go build` inside a main package drops its binary beside the source, and
+# `git add -A` will take it. One got as far as main at 18 MB (fa7c6c0) --
+# every clone of this repo carries it for good now. These are the main
+# packages whose directory name is also their binary name.
+/hack/demo-simulator/demo-simulator
+/hack/benchmark-fixture/benchmark-fixture
+/hack/alert-load-test/alert-load-test
+/hack/webhook-sink/webhook-sink
+/hack/mock-ollama/mock-ollama