feat: support setting and using a default organization

[betegon]

Summary

Currently, when org resolution fails, the error hint says "Set a default: sentry org view <org>" — but there's no mechanism to actually set a default org.

Proposal

• Implement sentry org default <org> (or similar) to persist a default org slug, and use it as a fallback when no org is specified or detected from DSN.
• Update the error hint in src/commands/project/create.ts:319 to reference the correct command once implemented.

Context

Reference: PR review comment from @BYK suggesting this as a follow-up feature.