[Bug]: Worktree threads can miss existing pull requests

[tonoizer]

Description

Worktree thread VCS status currently couples pull-request discovery to remote Git refreshes and treats lookup failures or an uninitialized remote snapshot like a confirmed absence. This can leave an externally created pull request undiscovered and expose an unsafe Create PR action.

Expected behavior

• Poll lightweight PR/MR metadata while a worktree thread is subscribed, even when automatic Git fetch is disabled.
• Correlate cached remote and pull-request state with the branch for which it was calculated.
• Represent pending and failed provider lookups explicitly.
• Refresh local and change-request status after primary agent turns.
• Only offer PR creation after a successful lookup confirms no open change request.

Scope

This should preserve existing provider filtering, default-branch handling, fork matching, and the server-side existing-PR safeguard.