errors: internal/errors is dead code; API errors reach users raw (handleError passthrough)

[timimsms]

Context

The internal/errors package (typed APIError/UserError, HandleHTTPError) is dead code: nothing invokes it, so raw API/HTTP errors reach users unformatted. Error handling style is also split across the command tree, which blocks consistent, structured error output.

Evidence

• internal/api/client.go:234 — Client.handleError is a passthrough:

// TODO: Parse HTTP response codes and convert to appropriate errors
// For now, return the error as-is
return err

• internal/errors — HandleHTTPError, APIError, and UserError are never invoked anywhere in the codebase.
• Command error style is inconsistent: 96 call sites use Run + fmt.Fprintf(os.Stderr, ...) + os.Exit(1), while others use RunE. The os.Exit paths skip cobra's error handling entirely.

Suggested fix

1. Wire HandleHTTPError into api.Client.handleError so HTTP status codes map to typed, user-friendly errors (401 → "run cu auth login", 404 → resource context, 429 → rate-limit guidance).
2. Migrate commands from Run+os.Exit to RunE so errors flow through a single formatting point.

This is a prerequisite for the structured JSON error output tracked in #18.