Add AGENT.md with project development guidelines (#497)

Amp-Thread: https://ampcode.com/threads/T-807f4928-eee4-49c2-b53a-a5cca9671ec4

Co-authored-by: Amp <amp@ampcode.com>

Author: sj26

AGENT.md

@@ -0,0 +1,18 @@
+This project is the Buildkite CLI (`bk`)
+
+## Commands
+- Test: `go test ./...`
+- Lint: `docker-compose -f .buildkite/docker-compose.yaml run lint`
+- Generate: `go generate` (required after GraphQL changes)
+- Run: `go run cmd/bk/main.go`
+
+## Environment
+- `BUILDKITE_GRAPHQL_TOKEN` required for development
+
+## Project Structure
+- Main binary: `cmd/bk/main.go`
+- GraphQL schema: `schema.graphql`
+- CLI commands: `pkg/cmd/`
+
+## Notes
+- CI: https://buildkite.com/buildkite/buildkite-cli