feat: Add one-step context setup with env export

[YoungJinJung]

Summary

Add a one-step CLI workflow that lets users select an AWS context/account, activate it, and immediately emit shell exports for reuse in aws-cli and other local tools.

Why

UNIC currently lets users pick or create contexts inside the TUI, but users also need a smooth way to carry the selected auth into aws-cli, Terraform, and related terminal tools. The best UX here is a single interactive command that switches context and prints export commands that can be consumed with eval "$(unic context setup)".

Scope

• Add unic context setup for interactive context selection/setup
• Support configured SSO, assume-role, and credential contexts in one flow
• Reuse an existing SSO portal configuration to log in and list accessible AWS accounts/roles when the selected base context uses SSO
• Save or reuse a matching UNIC context and set it as current
• Print shell export commands as the command output so the command can be used directly with eval
• Keep the flow compatible with temporary credentials and avoid mutating ~/.aws/credentials
• Keep unic env as a non-interactive follow-up command for the current or named context

Checklist

• Add CLI commands for context setup and env
• Make context setup activate the selected context and emit shell exports on stdout
• Send prompts/status text to stderr so eval "$(unic context setup)" stays usable
• Implement SSO account/role discovery using the AWS SSO APIs
• Prompt for context, account, and role selection in the terminal as needed
• Save or reuse the selected context in ~/.config/unic/config.yaml
• Print export commands for shell usage without leaking secrets to logs
• Add tests for config upsert, env rendering, and command wiring
• Update README usage docs