feat(nixvim): follow LazyGit worktree switches

[dc-tec]

Summary

• enable LazyGit through Nixvim's native plugin module
• use LazyGit's directory handoff to detect repository and worktree switches
• open a switched worktree in a new tab rooted at that checkout, with the project dashboard
• preserve existing callbacks and clean up the handoff on failed terminal exits and editor shutdown

Root cause

LazyGit runs as a child process of Neovim. Switching worktrees changes the child's working directory, but a child process cannot change Neovim's working directory, so the editor remained rooted in the original checkout.

User impact

Selecting another worktree in LazyGit and exiting with q now opens that checkout in a new editor tab while preserving the original tab. Exiting with Q keeps the original worktree.

Validation

• nix flake check path:. --print-build-logs
• generated Nixvim package build
• headless runtime simulations for changed, unchanged, and failed LazyGit exits
• environment restoration and Snacks dashboard assertions
• manual nix run path:. -- --cmd 'cd <worktree>' verification
• git diff --check