Fix lint errors

[pglass]

What was changed

• Fix all lint errors
• Enable golangci-lint in PR checks

Why?

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

[temporal-nick]

FYI, build failed:

go get -tool -modfile=develop/tools.mod golang.org/x/tools/cmd/goimports@latest
flag provided but not defined: -tool
usage: go get [-t] [-u] [-v] [build flags] [packages]
Run 'go help get' for details.
make: *** [Makefile:3[7](https://github.com/temporalio/s2s-proxy/actions/runs/15831199136/job/44624016012?pr=100#step:4:8): update-tools] Error 2

[pglass]

Switched to https://github.com/daixiang0/gci for code formatting. I've seen a couple other teams use this. It does a better job of grouping imports than goimport does.

goimports doesn't merge two import groups if they are adjacent, and it allows import groups to be in any order

import (
	"github.com/hashicorp/yamux"
	"go.temporal.io/server/common/backoff"

	"github.com/temporalio/s2s-proxy/config" // <<<

	"github.com/temporalio/s2s-proxy/encryption" // <<<
	"github.com/temporalio/s2s-proxy/metrics"

	"fmt"
	"net"

While with gci, it will sort and merge into a minimum number of import groups:

import (
	"fmt"
	"net"

	"github.com/hashicorp/yamux"
	"go.temporal.io/server/common/backoff"

	"github.com/temporalio/s2s-proxy/config"
	"github.com/temporalio/s2s-proxy/encryption"
	"github.com/temporalio/s2s-proxy/metrics"

[pglass]

Switch to use golangci-lint fmt, which supports a few different standard formatters out of the box (so one fewer thing to install): https://golangci-lint.run/usage/formatters/#goimports