Update SKILL.md with additional code style guidelines

[gewarren]

Clarify guidelines on string interpolation and expression-bodied members.

Contributes to #12713.

[Copilot]

Pull request overview

This PR updates the dotnet-snippet-modernization agent skill documentation to clarify when to prefer string interpolation vs. composite formatting, and how to format expression-bodied members for readability, aligning snippet modernization with repository formatting expectations.

Changes:

• Adds guidance to avoid string interpolation when it would reduce readability (with an illustrative example).
• Clarifies formatting guidance for long expression-bodied members (with an example).
• Adds an explicit curly-braces formatting guideline.

Suppressed comments (1)

.agents/skills/dotnet-snippet-modernization/SKILL.md:39

• This bullet uses "for example" twice in the same sentence, which reads awkwardly. Removing the second usage keeps the guidance clearer.

- Use expression-bodied members for simple single-expression members. If the line gets too long, for example, a method signature with a type parameter constraint, place the expression body on a new line, for example:

[wadepickett]

@gewarren, approved. Looks good.