chore(lint): enable perfsprint and usetesting, and apply their rewrites

[otavio]

What

Enables perfsprint and usetesting, and applies the ~200 rewrites they report.

Why

Third step of the lint expansion, after #6980 and #6981. Both linters carry autofixes, so almost every hunk here was written by the tool rather than by hand — review the diff, not the reasoning behind each line.

• perfsprint — fmt.Sprintf used where concatenation or strconv does the job, fmt.Errorf with no format verbs where errors.New belongs, and fmt.Sprintf("%x", b) where hex.EncodeToString is both clearer and faster.
• usetesting — t.Setenv, t.Context() and t.TempDir() over the hand-rolled equivalents.

Changes

One file needed hands. pkg/envs/envs_test.go set the environment through a before closure and undid it through a matching after, neither of which takes a *testing.T, so the fix could not be mechanical. Those cases now take one and call t.Setenv, which restores the previous value when the subtest ends. That is exactly what the after closures did by hand, so they are gone — 4 test tables, ~40 lines of teardown deleted.

Everything else is the linter's output, plus one tidy-up: perfsprint left five "/api/devices/" + "3a47..." pairs of adjacent literals, which I merged.

Not included

dupword was in this group and is deliberately left out. Its autofix corrupts test fixtures, because in fixture data the repetition is the data. Applied to this tree it:

• rewrote a /proc/self/mountinfo sample in pkg/dockerutils/utils_test.go, stripping the filesystem type where the format legitimately repeats it (- overlay overlay rw,... became - overlay rw,...)
• turned VALUES (..., now(), now(), ...) into now() in three migrations_test.go inserts, changing the statement's column count, and mangled a string concatenation into ,"+ownerID+' in the process

Both would have compiled. Neither is a change anyone would approve on purpose.

Testing

Under golang:1.26.7-alpine3.24:

• golangci-lint run ./... reports 0 issues for all six modules
• go build ./... clean everywhere, plus agent under -tags docker and -tags native
• go test ./... passes for root, server, and agent under -tags docker
• go mod tidy is a no-op

https://claude.ai/code/session_01D4BSojj4fmh3ZQSZWkGbD8

[github-actions]

Claude finished @otavio's task in 1m 28s —— View job

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Code Review Complete

Reviewed 24 files across code quality, security, testing, language patterns, and architecture — no new issues found. The code looks good as-is.

This is a mechanical perfsprint + usetesting autofix pass, and the rewrites hold up:

• Import consistency — spot-checked the files where an autofix can strand or drop a fmt/errors import (server/ssh/web/session.go, server/ssh/session/session.go, server/ssh/web/errors.go, server/ssh/session/errors.go, pkg/revdial/revdial.go, agent/server/session.go). All consistent — fmt is retained only where still used, errors is imported everywhere it's now referenced.
• Semantic equivalence — hex.EncodeToString(buf) ≡ fmt.Sprintf("%x", buf) for []byte, strconv.Itoa(version) ≡ fmt.Sprintf("%d", version) for int, and the errors.New conversions preserve errors.Is identity semantics for the package-level Err* vars (no %w was present in any of them).
• envs_test.go t.Setenv conversion — safe: none of these tests call t.Parallel(), and per-subtest restore is actually stricter than the hand-rolled after closures it replaces (some of which unset the wrong keys).
• tests/ssh_test.go — os.CreateTemp(t.TempDir(), ...) is correct; the leftover defer os.Remove(...) is now redundant but harmless.

Good call excluding dupword — its autofix genuinely corrupts fixture data, and the reasoning in the description is sound.

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· chore/lint-idiom