Install golangci-lint and fix lint issues

[riccardo-perotti]

Installs golangci-lint in CI and fixes all reported issues (~85, across 22 files). Lint clean (0 issues); go build, go vet, and tests pass. Pure lint-fix — zero runtime behavior change.

CI

Added a lint job to .github/workflows/build.yml (golangci/golangci-lint-action@v6, Go 1.26), alongside the existing build job and reusing its actions/setup-go@v5 setup. No .golangci.yml — default linters, matching the sibling rollout repos.

Fixes by category

• errcheck (28+): _ = / _, _ = on intentionally-ignored returns (Write, Flush, Close, Register, RemoveClient, Output, Gettimeofday, deferred closes, etc.).
• SA1019: io/ioutil → os.ReadFile (service.go, ticket.go, scamp.go, tests).
• ST1005: lowercased capitalized error strings (interpolated values unchanged) across connection.go, requester.go, packetheader.go, mocked.go.
• staticcheck simplifications: if/else if chains → tagged switch (connection.go packetType, requester.go Envelope); make(x,n,n) → make(x,n); s[a:len(s)] → s[a:]; buffer.Bytes()/string(buffer.Bytes()) → buffer.String(); strings.Replace(…,-1) → ReplaceAll; removed redundant returns in void funcs and an empty else.
• ineffassign: fixed two ineffectual assignments in packet.go ReadPacket.
• unused / dead-code removal (17): unused vars/fields/funcs/types — incl. a main() defined inside package scamp (never an entrypoint) + its only callers (doFakeDiscoveryCache, doCertFingerprint) in scamp.go; an unstarted periodic-stats loop (serviceStats/gatherStats/printStatsLoop, stats.go); unused test helpers. No exported API removed; TLS/dial/wire-protocol paths untouched.
• S1000: for { select { case m, ok := <-ch } } → for m := range ch in client.splitReqsAndReps (range over a channel exits on close exactly as the old !ok branch did — behavior-identical).

Deferred — two pre-existing latent bugs (NOT fixed here; tracked in a separate ticket)

golangci-lint surfaced two genuine latent bugs. Per the behavior-preservation mandate they are left exactly as-is (the dead assignments the linter flagged were removed without changing behavior), and filed separately:

1. serviceproxy.go newServiceProxy — malformed announce data can panic (index out of range): length checks set err but don't return. Original (panicking) behavior preserved.
2. connection.go Send — write errors on a closed/broken connection are swallowed (returns nil = success). SA4006 flagged the dead assignment; removing it preserves the original swallow-on-error behavior exactly.

Ticket created for the above issues: https://gudtech.atlassian.net/browse/ROP-9245

Jira Tickets

https://gudtech.atlassian.net/browse/ROP-9232

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