feat(nixvim): add VCS and language tooling

[dc-tec]

Summary

• add an asynchronous Lualine VCS segment for Git, Jujutsu, and GitHub/GitLab/Tangled/Radicle remotes
• add Jujutsu commands and a complete Go workflow with formatting, tests, and debugging
• add modular OCaml, Nix, OpenTofu/HCL, YAML/Helm, and Rust support
• prefer project-local toolchains where version coupling matters and keep generic HCL schema-neutral
• expose nixfmt-tree as the flake formatter so nix fmt formats the repository correctly

Why

The editor configuration had broad LSP coverage but lacked the project-aware workflows used most often in this repository set. The statusline also exposed only Git context, which made colocated Jujutsu work and multi-forge repository state invisible.

Impact

The generated Neovim package now provides language-aware LSP, parsing, formatting, testing, and debugging integrations while preserving local opam, Dune, rustup, dev-shell, and OpenTofu toolchain precedence. GitHub remains the canonical integration surface; Tangled and Radicle can mirror the exact branch and eventual canonical result without independently advancing main.

Validation

• nix fmt
• nix flake check on aarch64-darwin
• pre-commit nixfmt and statix hooks
• headless runtime assertions covering filetypes, Tree-sitter parsers, LSP registration, formatters, SchemaStore, keymaps, Git/Jujutsu status, and forge detection