chore: replace bingo with Go 1.24 tool directives

Remove bingo tool management in favor of native `tool` directives
in go.mod (Go 1.24+). Tool versions are pinned in go.mod/go.sum
and resolved via `go tool <name>`.

Pinned versions (unchanged from bingo):
- golangci-lint v2.7.0
- oapi-codegen v2.6.0
- mockgen v0.6.0
- gotestsum v1.13.0

Author: kuudori