Structure errors as Title/Reason/Status/Next step, name exit codes

[DO-rrao]

Summary

• checkErr now resolves every error to a StructuredError (an explicit implementation, or a genericStructuredError synthesized from a new status-code table) and renders Title → Reason → Status → Next step as separate lines, mirroring the same four fields into the JSON error envelope additively (detail unchanged for existing consumers).
• Adds commands/error_codes.go: canned reason + a run <command> next step for common API status codes (400/401/403/404/409/422/429/5xx), so Style.Hint bolds the whole suggestion consistently.
• FlagValidationError implements the new interface for its JSON output while keeping its own richer per-flag text block untouched.
• Names the two exit codes doctl has always used (exitGeneralError = 1, exitUsageError = 255) instead of magic numbers, and routes the config-read failure through checkErr instead of a raw log.Fatalln, so it gets consistent formatting and a controlled exit code.
• Fixes a pre-existing compile bug in auth.go's writeConfig(), which referenced an out-of-scope context identifier; it's now a parameter from its one call site.

Test plan

• go build ./cmd/doctl succeeds
• go vet ./commands/... ./internal/... clean
• go test ./commands/... — existing checkErr/FlagValidationError tests pass unchanged (one pre-existing, unrelated failure: TestRenderWelcomeUsesASCIIGlyphsWhenRequired, confirmed to fail identically without these changes)
• Manually verified: 404/text, 404/JSON, missing-flags/text, missing-flags/JSON, unknown command (exit 255), success path (exit 0), corrupt config file (exit 1, styled instead of raw crash), forced-PTY color/bold rendering

🤖 Generated with Claude Code

[SSharma-10]

We are printing the success message twice: notice above and fmt.Printf in writeConfig L362

[SSharma-10]

Build looks good now, but the auth switch is still broken.

Locally, go test ./commands/ is failing with context does not exist in:

• TestAuthSwitchPersistsOnlyTheContext
• TestAuthForcesLowercase

Looks like RunAuthSwitch is still checking viper while writeConfig checks the YAML file, so they are out of sync. Can you wire the switch through the file-based path as well?

[SSharma-10]

Tried this locally:

doctl compute droplet get 999999999

Text only shows "Not Found" + the canned reason, the actual API message only shows up in json detail.
Also for a 404, suggesting doctl auth list feels off