New 'Claude Design' sign-in flow for host and CLI sessions

What

• A new host-side sign-in flow called "Claude Design sync" opens a browser page for sign-in, polling for completion and streaming session events.
• Manual authorization-code entry now works even partway through the flow, not just as an unconditional fallback for remote sessions.
• Failures now report a structured reason code (interrupted, unconfigured, remote, invalid, save_failed, timed_out, failed) instead of just a free-text message.
• A new hidden CLI command, claude design-login --json, runs this flow (or reports status with --status) as JSON lines, meant for tools like the VS Code extension rather than everyday interactive use.

Why This gives Claude Code, and tools built on it such as the VS Code extension, a reliable, scriptable way to sign in through a browser, with clearer error reporting when sign-in fails and a way to enter a code by hand when the browser flow can't complete automatically.