[Bug] /design-login fails to grant design scopes despite valid subscription account

Bug Description
Title: /design-login fails to grant design scopes — DesignSync returns 400 despite valid subscription account

Body:

DesignSync (design-sync workflow) cannot authorize even though the account has design access.

Environment:

• Model: Opus 4.8 (claude-opus-4-8[1m])
• Platform: darwin 24.6.0, zsh
• Account: has design access confirmed — claude.ai/design shows the project list/creation UI in the browser

Steps to reproduce:

1. In a Claude Code session, trigger a DesignSync read call (e.g. list_projects)
2. Get error prompting to run /login or /design-login
3. Run /login → "Login successful"
4. Run /design-login
5. Retry list_projects

Expected: design scopes added to token; list_projects returns writable design-system projects.

Actual: Every DesignSync call returns:
DesignSync needs a claude.ai login. Could not add design scopes to the token.
Run /login ... or run /design-login to authorize design access separately.
(Request failed with status code 400)

Additional notes:

• /design-login does not open a browser and produces no visible authorization flow / URL / output.
• The account demonstrably has design access (claude.ai/design is usable in the browser), so this is a token-scope grant failure in the CLI, not an account-permission issue.
• Mid-session /login did not refresh the token with design scopes.

Environment Info

• Platform: darwin
• Terminal: WarpTerminal
• Version: 2.1.197
• Feedback ID: d0ca728a-4f56-4bb6-bcc0-62fb4f1fe3dc

Errors

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