feat(cli): add init provider flag

[terminalchai]

Summary

• add a --provider flag to skrun init and map it to the default model for each supported provider
• skip the interactive model prompt when --provider is supplied while still letting --model take precedence
• document the new init flow and cover it with CLI package tests

Testing

• pnpm build
• pnpm test
• pnpm exec biome check README.md docs/cli.md packages/cli/src/commands/init.ts packages/cli/src/commands/init.test.ts packages/cli/src/utils/prompts.ts

[Tarcroi]

Thanks for the PR! Looking good at first glance — will review in detail later this week.