tooling: adopt go tool directives; rename air→dev; bundle fmt drift
- Justfile: rename `air:` to `dev:`, body uses `go tool air`; the three
migrate recipes use `go tool migrate`; `fmt` adds `go fix ./...`.
Stale `brew install golang-migrate` comment block replaced with the
one-line `go get -tool` bootstrap hint.
- README.md: quickstart says `just dev`.
- go.mod: declare github.com/air-verse/air and golang-migrate's cmd
in the `tool` block (Go 1.24+) so contributors never need a separate
install step. Updates go.sum accordingly.
- internal/domain/stats/repository.go: pre-existing `HostSplit` indent
drift + trailing newline, surfaced by goimports inside the new fmt
recipe.