docs(tac): add reproducible agentic dev environment appendix

Folds kunchenguid dotfiles architecture (Nix + Home Manager + symlinks
+ rebuild.sh) into loop_engineering.md as a cross-OS reference. Covers
the Windows port sketch (scoop/winget, Windows Terminal, Herdr native)
and the key insight: agent configs as versioned symlinks from one
source-of-truth agents.md file.

Reference: youtube.com/watch?v=5N-okeDdIuI, github.com/kunchenguid/dotfiles

plans/meta-prompts/loop_engineering.md

@@ -189,3 +189,28 @@ TAC invariants still hold when a multiplexer is the substrate:
 - the primary agent must not gain a raw-command deploy path just because it can drive shells.
 
 References: cmux fleet-driving prompts (`github.com/disler/learning-cmux-with-agents`), herdr agent multiplexer (`herdr.dev`), cmux + Claude Code + Pi demo (YouTube `WAFUMBLOjHo`).
+
+## Reproducible agentic dev environment
+
+Reference: kunchenguid dotfiles (`github.com/kunchenguid/dotfiles`), Agentic Engineering Setup walkthrough (YouTube `5N-okeDdIuI`).
+
+**Architecture stack:** Nix (Determinate Nix) → Nix Darwin (macOS system) / Home Manager (user-level packages + dotfiles) → symlinks (agent configs, global agents.md) → `rebuild.sh` (one command from fresh OS to productive setup).
+
+**Key insight — agent configs as versioned symlinks.** The global `agents.md` memory file lives in the dotfiles repo. Home Manager symlinks it into each agent's config directory. All agents (Claude Code, pi, etc.) read the same ruleset from one source-of-truth file. Change once, rebuild, every agent picks it up.
+
+**Cross-OS portability (Windows sketch):**
+
+| macOS component | Windows equivalent |
+|---|---|
+| Nix + Nix Darwin | Nix via WSL, or scoop/winget |
+| Homebrew | scoop / winget / chocolatey |
+| Westerm terminal | Windows Terminal / WezTerm |
+| Zsh + Starship | PowerShell Core + Starship, or Git Bash |
+| Neovim (Lazy.nvim) | Neovim (identical, paths differ) |
+| Herdr multiplexer | Herdr (native Windows binary) |
+| Claude Code | Claude Code (Windows CLI) |
+| `rebuild.sh` | `rebuild.ps1` or cross-OS `justfile` |
+
+**Cross-OS constants** that make this viable: Herdr (single Rust binary, works everywhere, see Agent multiplexer note above); Neovim (identical config across OSes); Claude Code/pi (both work on Windows); Starship (cross-shell prompt); global `agents.md` (plain markdown, OS-agnostic).
+
+**The target pattern:** one command from a fresh OS install to a fully productive agentic engineering environment. Every tool, every config, every agent rule — versioned, repeatable, documented for the OSes you target.