Add ability to create worktrees from a starting point (linear issue, pull request, etc.)

[jakeleventhal]

In conductor, you have the ability to create a new workspace with ⌘+⇧+N to give you options to start a new worktree from either

1. Linear issue
2. GH pull request
3. existing branch

This makes it much easier to spawn a task with the proper context and starting point without having to click around.

PROMPT SUGGESTION

Add a new way to spawn a thread using ⌘+⇧+N that spawns a window pane with 3 tabs:

1. Pull requests
2. Branches
3. Issues
   Illustrated below:

Pull requests will require a GitHub integration with your repository on the settings page.
This option should create a worktree from that branch and should load all the PR description and comments into context by default.

Linear issues will require a Linear integration with your repository on the settings page.
This option should create a worktree from that branch and should load the entire linear issue context into the thread.