feat(fish): add jj-aware Tide VCS segment

[nvonbulow]

Summary

• add a custom Tide vcs segment for Jujutsu repositories
• hide Git segment in jj repos by replacing git with vcs in tide_left_prompt_items
• show shortest unique jj change id
• show nearest bookmark context with color semantics (current/parent/child)
• add octicon-based change counters (added/modified/removed/renamed/copied/conflicts), hidden when zero
• keep colors aligned with existing Tide git colors

Validation

• fish -n home/dot_config/fish/functions/_tide_item_vcs.fish
• fish -n home/dot_config/fish/conf.d/00-tide-vars.fish
• manual prompt checks in jj and non-jj directories

Notes

• This branch builds on existing fish/tide setup commits that are not on master yet.