chore: inject version via ldflags instead of hardcoding

Version string in `main.go` is hardcoded as `v0.9.0`. Should be injected at build time via `-ldflags` so it stays in sync with git tags.

Pattern:
```go
var version = "dev"
```

```makefile
LDFLAGS := -s -w -X main.version=$(shell git describe --tags --always)
```