lint: clean up 75 pre-existing golangci-lint v2.12.2 issues

[MikeBengtson]

Summary

CI was failing on main with 75 golangci-lint issues across the codebase. This PR drives the count to 0.

Resolution:

• gofmt -w on 35 files (auto-formatting drift)
• golangci-lint run --fix resolved staticcheck QF/S simplifications
• Manual fixes for: bodyclose (resp.Body.Close), errcheck (`_ = ...`), staticcheck (TrimPrefix, ST1005 punctuation, SA1012 nil context, SA9003 empty branch, SA4017 nolint with reason), gocritic (appendAssign with explicit make+append), unused (nolint with reason for build-tag-gated symbols)

Notable nolint usage (each with explicit reason in code):

• `internal/adapter/firecracker/firecracker.go` — `egressTable`, `errAlreadyStopped` referenced only by `firecracker_linux.go`
• `internal/server/router.go` — `orgGates` consumer lands with M4 Workstream B + M2 wsid registry: quota, OAuth, billing, admin, egress CLI, workspaces #9
• `internal/sse/sse.go` — two empty switch cases (intentional per SSE spec)

Test plan

• `go build ./...` clean
• `go test ./...` — 3816 pass in 125 packages
• `golangci-lint run --timeout=2m` — 0 issues

Once this merges, PR #9 should pass CI on rebase.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com