🤖 fix: ignore generated coder-k8s build artifact (#47)

## Summary
Ignore the generated repository-root binary (`./coder-k8s`) so local
builds do not leave a dirty working tree.

## Background
`make build` writes the project binary to `./coder-k8s`, but that
artifact was not listed in `.gitignore`.

## Implementation
- Added `/coder-k8s` to `.gitignore` under the Go artifacts section.

## Validation
- `make verify-vendor`
- `make test`
- `make build`
- `make lint`
- `git check-ignore -v ./coder-k8s`

## Risks
Very low. This only affects git ignore behavior for a generated local
build artifact.

---

_Generated with `mux` • Model: `openai:gpt-5.3-codex` • Thinking:
`xhigh` • Cost: `$0.10`_

<!-- mux-attribution: model=openai:gpt-5.3-codex thinking=xhigh
costs=0.10 -->

Author: ThomasK33

.gitignore

@@ -1,5 +1,6 @@
 # Go
 /bin/
+/coder-k8s
 *.test
 *.out