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

[axisrow]

Closes #613

Problem

Branch and worktree names render at full length, so a status line carrying ⎇ fix/np/story-123 𖠰 dir/wt spends a lot of width on segments that mostly serve as a prefix. The Current Working Dir widget already has a fish-style mode that collapses path segments, but the git name widgets have no equivalent — and their default rendering is inconsistent with a fish-style cwd next to them.

Fix

Adds a per-widget fish-style mode to both Git Branch and Git Worktree (off by default, editor keybind (f)ish style, stored as metadata.fishStyle):

• every name segment but the last collapses to its first character: main → m, feature/worktree → f/worktree, fix/np/story → f/p/story, worktree dir/wt → d/wt;
• the TUI preview and the editor modifier text reflect the mode;
• the repo link keeps the full branch name in the URL — only the visible label is abbreviated;
• maxWidth still applies on top of the abbreviated text.

Verification

• New tests in GitBranch.test.ts (plain and slashed branches, editor modifier + keybind) and GitWorktree.test.ts (main and nested worktrees, editor modifier + keybind).
• Full suite: 2364 tests pass; bun run lint clean.
• Live check against a real build: line renders ⎇ m 𖠰 m with both toggles enabled, ⎇ f/git-branch-fish-style on a long-running branch name.

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