Add multi-instance support with --rid flag for managing multiple Redmine servers

[Copilot]

Enables simultaneous management of multiple Redmine servers. Users can now login to different instances and switch between them using --rid <instance-id> without repeated authentication.

Changes

Configuration

• Extended config structure to store multiple instances in a map keyed by instance ID
• Added MultiInstanceConfig_t with instance map and default instance tracking
• Maintains backward compatibility: legacy single-instance configs load automatically without migration

CLI

• Added global --rid persistent flag to all commands
• Flag parsing in PersistentPreRunE before command execution
• Instance selection: commands without --rid use default, with --rid target specified instance

Authentication

• auth login --rid <id>: stores credentials for specific instance
• auth logout --rid <id>: removes only specified instance credentials
• First login becomes default instance automatically

Testing

• 7 new unit tests for multi-instance config operations
• 1 new test for login with --rid flag
• All existing tests pass (backward compatibility verified)

Usage

# Login to multiple instances
red-cli auth login --rid prod
red-cli auth login --rid staging

# Target specific instance
red-cli issue list --rid staging
red-cli issue create --project 42 --rid prod

# Logout specific instance
red-cli auth logout --rid staging

Config Format

{
  "instances": {
    "prod": {
      "server": "https://prod.redmine.example.com",
      "api-key": "...",
      "user-id": 1,
      "project-id": 23
    },
    "staging": {
      "server": "https://staging.redmine.example.com",
      "api-key": "...",
      "user-id": 5,
      "project-id": 42
    }
  },
  "default-instance": "prod"
}

Documentation

• Created comprehensive multi-instance guide with examples and use cases
• Updated README, config guide, and auth command documentation
• Added examples for common workflows (multiple clients, prod/staging environments)

Original prompt

The feature request is to enable the redmine-cli to support multiple Redmine instances. This includes adding the functionality to login to multiple Redmine instances and use them from the command line. A Redmine instance should be identified by a Redmine ID passed as an argument, such as --rid 2.

To implement this:

• Update the login functionality to allow managing credentials for multiple instances.
• Adjust the command-line parsing to recognize and handle the --rid argument.
• Add support for selecting the desired Redmine instance based on Redmine ID.
• Ensure backward compatibility, so current single-instance usage still works seamlessly.
• Write tests to validate the functionality of multi-instance support.
• Update documentation to explain how to configure and use multiple Redmine instances, including examples of command usage.

This feature should align with milestone v0.2.0.

This pull request was created as a result of the following prompt from Copilot chat.

The feature request is to enable the redmine-cli to support multiple Redmine instances. This includes adding the functionality to login to multiple Redmine instances and use them from the command line. A Redmine instance should be identified by a Redmine ID passed as an argument, such as --rid 2.

To implement this:

• Update the login functionality to allow managing credentials for multiple instances.
• Adjust the command-line parsing to recognize and handle the --rid argument.
• Add support for selecting the desired Redmine instance based on Redmine ID.
• Ensure backward compatibility, so current single-instance usage still works seamlessly.
• Write tests to validate the functionality of multi-instance support.
• Update documentation to explain how to configure and use multiple Redmine instances, including examples of command usage.

This feature should align with milestone v0.2.0.

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