Add httperr package for HTTP status code-aware errors

[JAORMX]

Summary

• Graduate the errors package from toolhive as httperr - a more descriptive name for HTTP status code-aware error handling
• Second Tier 1 package per THV-0032 RFC
• Provides CodedError type that carries HTTP status codes through the error chain
• Full support for errors.Is() and errors.As() via Unwrap()
• Comprehensive package documentation with usage examples

API

// Create a new error with status code
err := httperr.New("not found", http.StatusNotFound)

// Wrap existing error with status code
err := httperr.WithCode(err, http.StatusBadRequest)

// Extract status code from error chain
code := httperr.Code(err) // Returns 500 if no CodedError found

Test plan

• task lint passes
• task license-check passes
• task test passes with race detection

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