ci: stop tracking the built binary

`go build ./...` writes the binary into the package directory under the
package's own name, so a routine build followed by `git add -A` committed
12MB of darwin/arm64 Mach-O to a repo whose image is built linux/amd64
from source by the Dockerfile. It has been dead weight in the build
context ever since, and `COPY . .` was shipping it to the builder only
for `go build` to overwrite it.

The artifact is never an input. Only the source is.

Co-authored-by: Hanzo Dev <dev@hanzo.ai>

.gitignore

@@ -0,0 +1,5 @@
+# `go build ./...` writes the binary into the package directory, named after the
+# package — which is how a 12MB darwin/arm64 `ci` came to be committed here, in
+# a repo whose image is built linux/amd64 from source by the Dockerfile. The
+# artifact is never an input to anything; only the source is.
+/ci