style: adopt golangci-lint v2 and resolve all findings

Add .golangci.yml (v2): the standard linters plus bodyclose, errorlint, noctx, misspell, rowserrcheck, sqlclosecheck, unconvert, usestdlibvars, with gofumpt + goimports formatters. Enable the std-error-handling exclusion preset for idiomatic Close/Remove/Setenv ignores; scope-exclude SA1019 (parser.ParseDir in tools/openapigen) and ST1005 (intentional capitalized user-facing error copy that tests assert verbatim). No inline nolint directives were introduced.

Resolve all 217 findings behavior-preserving: gofumpt/goimports formatting, explicit blank assignment on intentionally ignored errors, errors.Is/errors.As and %w wrapping, context-aware stdlib calls (CommandContext/QueryContext/NewRequestWithContext/Dialer), staticcheck simplifications, removed redundant conversions, http.StatusOK and http.MethodGet, inlined the go:fix intPtr helper, and deferred sql rows Close. Add a golangci CI job mirroring the existing Go jobs.

internal/util/sys/sys_windows.go

@@ -66,7 +66,8 @@ func CPUPercentRaw() (float64, error) {
 		uintptr(unsafe.Pointer(&userFT)),
 	)
 	if r1 == 0 {
-		if errno, _ := e1.(syscall.Errno); errno != 0 {
+		var errno syscall.Errno
+		if errors.As(e1, &errno) && errno != 0 {
 			return 0, errno
 		}
 		return 0, errors.New("GetSystemTimes failed")