dev: replace deprecated binutils-gold with CGO_ENABLED=0 in dev images

[gustavosbarreto]

Context

PR #6248 added binutils-gold to 6 development Dockerfiles to fix
ARM64 (Apple Silicon) builds failing with cannot find 'ld' during
go install of dev tools (air, golangci-lint, mockery).

The fix works but has issues:

1. Wrong root cause: the PR attributed -fuse-ld=gold to
   golangci-lint, but it actually comes from Go's own toolchain
   (/usr/local/go/pkg/tool/linux_arm64/link) when CGO is enabled
   on linux/arm64. Verified by reproducing in
   golang:1.25.8-alpine3.22 ARM64 container.

2. gold is deprecated: removed/marked deprecated in binutils 2.44
   (Jan 2025). Alpine will drop the package eventually.

3. Bloat for x86_64 users: the package is only needed on ARM64,
   but was added unconditionally to all dev images.

4. CGO not actually needed: the tools installed (air, golangci-lint,
   mockery) are pure Go and don't require cgo.

Reproduction

# Fails on ARM64 without binutils-gold
docker run --rm --platform linux/arm64 golang:1.25.8-alpine3.22 sh -c '
  apk add --no-cache git build-base
  CGO_ENABLED=1 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.3
'
# → /usr/local/go/pkg/tool/linux_arm64/link: running gcc failed: exit status 1
# → collect2: fatal error: cannot find 'ld'

# Works on ARM64 without binutils-gold
docker run --rm --platform linux/arm64 golang:1.25.8-alpine3.22 sh -c '
  apk add --no-cache git
  CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.3
'
# → success, binary works

Proposed fix

Replace binutils-gold install + default cgo with CGO_ENABLED=0
on all go install of dev tools, in 6 Dockerfiles:

# Remove binutils-gold (and build-base where it was added only for this) from apk add
RUN apk add --update openssl ...

# Add CGO_ENABLED=0 to each go install
RUN CGO_ENABLED=0 go install github.com/air-verse/air@v1.62 && \
    CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.3 && \
    CGO_ENABLED=0 go install github.com/vektra/mockery/v2/...@v2.53.2

Files affected:

• agent/Dockerfile
• api/Dockerfile
• cli/Dockerfile
• gateway/Dockerfile
• openapi/Dockerfile
• ssh/Dockerfile

Future-proofing note

If we ever need cgo in dev tools or in the actual app build, the
modern replacement for gold is ld.lld (LLVM linker), available
via apk add lld. To force Go to use it on ARM64 alpine:
ENV CGO_LDFLAGS="-fuse-ld=lld".