Adds command to migrate from v1 to v2 format

[geofflamrock]

This PR is part of supporting a tree structure with stack, where a branch can have multiple child branches:

• Add support for tree structure in config file #252
  • Adds command to migrate from v1 to v2 format #263
    • Support tree structure in status command #262
      • Support tree structure when creating new branch #265
        • Support tree structure when adding branch to stack #270
      • Support tree structure when updating stack #264
      • Support tree structure when creating pull requests #266
      • Support tree structure when removing branch from stack #271
      • Support tree structure when cleaning stack #272

[Copilot]

Pull Request Overview

This PR introduces a new stack config migrate command to transition user configuration files from v1 to v2 format.

• Refactors the existing stack config command into a branch with open and new migrate subcommands
• Updates StackConfig.Save to create parent directories and centralize backup file naming
• Adds handler, settings, and tests for the migration command, plus README updates

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Stack/Program.cs	Replaces single config command with config open and config migrate subcommands
src/Stack/Help/CommandNames.cs	Adds Migrate constant for the new subcommand
src/Stack/Config/StackConfig.cs	Ensures config directory exists and introduces GetV1ConfigBackupFilePath()
src/Stack/Commands/Config/MigrateConfigCommand.cs	Implements the CLI command and handler for config migration
src/Stack.Tests/Config/FileStackConfigTests.cs	Updates tests to use the new backup file path helper
src/Stack.Tests/Commands/Config/MigrateConfigCommandTests.cs	Adds basic migration test
README.md	Documents the new config open and config migrate usage