Backlog: Standardize private helper analyzer policy in contributor guidance

[lipkau]

Problem\nAnalyzer rules for private helper functions can be non-obvious, leading to avoidable lint churn.\n\n## Proposal\nDocument a small policy for helper naming, ShouldProcess expectations, and approved suppression patterns in this codebase.\n\n## Acceptance criteria\n- Guidance explains when helper-level suppressions are appropriate.\n- Examples align with existing repository patterns.\n- Lint-related contributor friction is reduced.