feat(nixvim): add AI review workflow

[dc-tec]

Summary

• add a read-only CodeDiff workflow for working-tree, branch, history, and Jujutsu review
• add Trouble views for diagnostics, quickfix, location lists, and TODO triage
• add repo-aware Overseer checks for Nix flakes, Go modules, and Git whitespace
• improve GitSigns navigation and native diff alignment

Validation

• nix fmt
• git diff --check
• nix flake check path:. --no-write-lock-file
• packaged headless runtime assertions, task discovery, and generated task execution