Support discovered worktrees and continuing existing branches

[ryanbbrown]

Problem

Thread creation only lists worktrees that belong to active bb environments.
It does not discover persistent Git worktrees that users manage themselves.

New managed worktrees also always create a generated bb branch. Selecting an
existing remote branch uses it as a base instead of checking it out directly.
This makes reopening an existing pull request awkward.

Requested behavior

• List every Git worktree for the selected project and machine.
• Show its branch, path, and machine.
• Treat discovered user worktrees as unmanaged workspaces.
• Keep user worktrees after their threads are archived.
• Add New branch and Continue branch modes inside the existing branch picker.
• Continue a local or remote branch directly in a temporary managed worktree.
• Configure remote tracking so a normal push updates the existing branch.
• Remove the temporary worktree after its threads are archived.
• Preserve the continued branch during cleanup.
• Reject branches that another worktree already checks out.

This should not change the default managed-worktree cleanup policy.

Reference implementation

A complete implementation is available here:

ryanbbrown@499533b

It includes the app, server, daemon, SDK, CLI, plugin, guide, and test surfaces.

AGENT GENERATED: by GPT-5.6

[ryanbbrown]

Closing because the remaining external-worktree discovery and branch-continuation gaps are real, but they are no longer important to my BB-managed workflow. I am withdrawing this request.

[banjerluke]

Well, they're important to my workflow. 😏

Did my best to get a robust implementation ready for merging in my PR. It's specifically focused on showing existing worktrees in the picker along with bb-managed worktrees, allowing you to continue work in them.