feat(cli): prune local branches with no worktree

[lettertwo]

Background

When a stack of two or more PRs merges while a worktree has one of the stack's branches checked out, git workon prune removes that worktree and deletes its branch, then stops. The other branches in the stack stay behind as local branches, since prune only ever considered branches through the worktree that had them checked out.

This problem is compounded for gh-stack users: gh-stack does not configure upstreams on branches, and a squash merge makes it impossible to tell when a branch was merged. As a result, PR merge status is the only reliable signal that can reach a merged stack's leftovers, which to date has not factored into any pruning decisions.

Change

This PR adds branch-only rows to prune. Every local branch with no worktree (minus the default branch, protected globs, and any branch checked out in a worktree, including the default one) joins the candidate pool and is evaluated for the same signals as a worktree row: PrMerged always, RemoteGone under --gone, Merged under --merged.

• On by default. --no-branches or workon.pruneBranches = false opts out. --keep-branch drops branch rows entirely.
• Branch rows show as (no worktree) in dry-run and carry a no worktree, ... annotation in the picker. Prompts and the success line count worktrees and branches separately.
• --json entries gain "kind": "worktree" | "branch", and a branch row's path is null.
• Named mode matches a branch-only row by branch name, with the same "not prunable, use --force" rule for a healthy one.

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