feat: create a branch for a work item from any branch

[JacobAtchley]

Why

Branching for a work item always started from the repository's default branch, so a branch for work on a feature or hotfix line had to be made somewhere else.

What

• b opens a new branch screen in place of the status-line name prompt:
  • repository: preselected from the working directory, otherwise the first; j/k switches and reloads branches
  • name: prefilled from type, id and title, editable
  • from: fuzzy-filtered list of the repository's branches (refs/heads only), cursor on the default branch
  • tab/shift+tab between fields, enter creates, esc backs out
• b works on a work item's own pane too, including one reached from a pull request.
• The view that opened the screen reports the outcome and copies the checkout once; the work item list still syncs its row to Active either way.
• New azdo.Branches; RefHead and the status-line repo picker are removed, since each listed branch carries its commit.
• README updated.

Testing

• go vet ./... and go test ./... green.
• New tests cover the screen (defaults, filtering, repo switch and stale listings, validation, esc, prompting) and owner routing between the list and the item pane.
• Tried by hand against Azure DevOps from both the list and the item pane.

🤖 Generated with Claude Code