refactor(ai-providers)!: remove the Cursor provider and Grok sign-in

[datlechin]

Stacked on #3361.

Why

The Cursor provider could not call TablePro's tools, ran the Cursor CLI with --trust, and Cursor already reaches TablePro with full tools through the MCP server. Sign in with xAI went through a Grok CLI proxy with a loopback OAuth listener, an unofficial path that duplicated the ChatGPT sign-in code. xAI keeps working with an API key.

Change

• Removed the Cursor provider: API and CLI transports, sign-in service, settings UI, strings and tests.
• Removed Sign in with xAI: OAuth and PKCE client, loopback callback server, token store, Grok proxy transport, settings UI, strings and tests.
• xAI is an API key provider on the Responses transport it already used for keys.
• No migration. A saved Cursor provider is dropped on load by fix(settings): decode settings per field so one bad value keeps the rest #3361's per-row decode; an xAI provider without a key shows Not configured.

Verified

A decode test drops a stored Cursor row and keeps the rest; XAIRegistrationTests pins xAI to the Responses transport.

Not in this PR

• Grok sign-in tokens already in the Keychain are left there; nothing reads them.
• Docs: docs(features): update AI providers for Copilot lifecycle and removals docs#8.