fix(fish): prune gone branches without losing local work

[SebastienElet]

Summary

• prune deleted upstream branches only when the local tip still matches the last observed upstream tip
• remove only clean single worktrees while preserving local commits, tracked, untracked, ignored, hidden-index, and multi-worktree state
• preserve Git status and dry-run semantics, detect the primary branch dynamically, and exercise the wrapper on macOS and Linux CI

Verification

• fish --no-config fish/tests/git_wrapper_test.fish (14 scenarios, macOS 26.5.2, Fish 4.8.1, Git 2.50.1)
• fish --no-config --no-execute fish/config.fish fish/conf.d/git-wrapper.fish fish/tests/git_wrapper_test.fish
• fish_indent --check fish/config.fish fish/conf.d/git-wrapper.fish fish/tests/git_wrapper_test.fish
• bash -n scripts/git_main_branch
• shellcheck --severity=error scripts/git_main_branch
• YAML parse and git diff --check

Closes #77