feat(git): fish-style abbreviation for branch and worktree names

[axisrow]

Feature request

Fish-style abbreviation for the Git Branch and Git Worktree widgets: collapse every name segment but the last to its first character, the same way the Current Working Dir widget's fish-style mode shortens paths.

Current	Fish-style
⎇ main	⎇ m
⎇ feature/worktree	⎇ f/worktree
⎇ fix/np/story	⎇ f/p/story
𖠰 dir/wt	𖠰 d/wt

Motivation

• The full path widgets already have this mode (Current Working Dir → (f)ish style), so abbreviated branches would keep the status line visually consistent.
• Branch names like fix/np/story-123 are long; f/p/story-123 keeps the distinguishing tail while cutting most of the width.
• It would be off by default (a per-widget toggle, (f) in the widget editor, metadata.fishStyle), so nothing changes for existing setups.

Notes

• The repo-link feature keeps the full branch name in the URL; only the visible label is abbreviated.
• maxWidth continues to apply on top of the abbreviated text.

Happy to send a PR — a working implementation with tests is ready.