fix: login and credentials configuration

[saeidalz13]

Summary

• kscli login now accepts --url to set the API base URL (defaults to https://api-staging.knowledgestack.ai), making onboarding
  a single step instead of two
• Removed the settings environment command and environment presets (local/staging/prod) — auth is now simply an API key bound
  to a base URL
• Simplified credentials to a single file instead of per-environment files (.credentials_local, .credentials_staging, etc.)
• Removed get_current_environment() from config

Test plan

• kscli login --api-key defaults to staging URL
• kscli login --api-key --url persists custom URL
• kscli settings show displays resolved config without environment field
• kscli logout clears the single credentials file
• make pre-commit passes (lint, typecheck)