Support --continue for Copilot and improve session setup

[coneilen]

Summary

• Copilot --continue: Enable the --continue flag for Copilot sessions when resuming, matching the existing Claude behavior. Falls back to a fresh session if --continue fails.
• Folder dialog: Add createDirectory property to the folder picker so users can create new directories inline.
• Auto git init: Automatically create the working directory and initialize a git repo when a new/empty folder is selected, with a warning if git is not installed.

Test plan

• Resume a Copilot session and verify it launches with copilot --continue || copilot
• Resume a Claude session and confirm existing --continue behavior is unchanged
• Open the folder picker and verify the "New Folder" button is available
• Create a session with a non-existent path and verify the directory is created with git init

🤖 Generated with Claude Code