Add a new worktreePaths config key that enables linked worktree checkouts. When a repository matches a worktreePaths entry, pressing C or Space (checkout) creates a linked worktree named after the PR branch — rather than switching branches in the clone directory. Fixes #544.

How it works

• repoPaths → the clone directory where git worktree add gets run
• worktreePaths → a worktrees directory where linked worktrees are created as subdirectories, one per PR branch

Both must be configured for the same repository or template pattern. Example:

repoPaths:
  dlvhdr/gh-dash: ~/code/dlvhdr/gh-dash

worktreePaths:
  dlvhdr/gh-dash: ~/code/dlvhdr/gh-dash/.worktrees

Behavior

• New PR: Creates a linked worktree and checks out the PR branch inside it
• Existing worktree: Skips creation, copies the path to the clipboard
• Checkout failure: Automatically cleans up the partially created worktree
• Repo not cloned: Shows an error asking to clone the repository first
• Missing repoPaths: Shows an error explaining that both settings are required
• Command errors: stderr is now captured and included in error messages (previously only showed “exit status 1”)

On success, the linked-worktree path is copied to the clipboard.

Code/test/docs/config notes

• Extracts checkout logic into a shared common.CheckoutPR() function used by both the PR and Notification dashboards (previously duplicated)
• Passes on, to the UI, more-detailed error messages from gh/git for checkout failures (RunCommandFunc now captures stderr from gh/git)
• Adds user docs, and updates the config parser, JSON/YAML schemas, example configs, and golden test files to add worktreePaths
• Adds test coverage for all the new behavior and possible error/failure scenarios