docs: add Vim/Neovim setup guide

README.md

@@ -230,6 +230,8 @@ your-project/
 
 Most config files embed the same instructions directly. OpenCode is the exception — `.opencode/opencode.json` references `.mex/AGENTS.md` instead of embedding content. `mex setup` asks which tool you use and creates the appropriate config.
 
+Neovim users have their own guide: see [docs/vim-neovim.md](docs/vim-neovim.md) for Claude Code, Avante.nvim, Copilot.vim, and generic-plugin setups.
+
 ## Contributing
 
 Contributions welcome! See [CONTRIBUTING.md](CONTRIBUTING.md) for setup and guidelines.
@@ -240,4 +242,4 @@ See [CHANGELOG.md](CHANGELOG.md) for release history.
 
 ## License
 
-[MIT](LICENSE)
+[MIT](LICENSE)

docs/vim-neovim.md

@@ -0,0 +1,72 @@
+# Using mex with Vim / Neovim
+
+mex's scaffold is tool-agnostic — any AI plugin that can read a system prompt or config file can use it. This guide covers four common setups.
+
+## 1. Claude Code in Neovim's terminal
+
+Run Claude Code directly inside Neovim. This is the path with the least setup — mex already generates a `CLAUDE.md` that Claude picks up automatically.
+
+```bash
+# Inside Neovim:
+:term claude
+```
+
+Claude Code reads `CLAUDE.md` from the project root, so mex's instructions are applied without any plugin configuration. Run `mex setup` (option 1) once to create `CLAUDE.md`, and you're done.
+
+## 2. Avante.nvim
+
+[Avante.nvim](https://github.com/yetone/avante.nvim) supports a custom system prompt. Point it at `.mex/ROUTER.md` (the file mex uses to route AI tools to the right context):
+
+```lua
+require("avante").setup({
+  system_prompt = function()
+    local f = io.open(vim.fn.getcwd() .. "/.mex/ROUTER.md", "r")
+    if not f then return "" end
+    local content = f:read("*a")
+    f:close()
+    return content
+  end,
+})
+```
+
+Run `mex setup` with option 8 (None / other) — this keeps `.mex/` populated without copying a tool-specific config.
+
+## 3. Copilot.vim / copilot.lua
+
+GitHub Copilot for Neovim reads `.github/copilot-instructions.md` automatically. mex already supports this via `setup.sh` option 4:
+
+```bash
+./setup.sh
+# Choose option 4 when prompted
+```
+
+No plugin config required — Copilot picks up the file as soon as it exists.
+
+## 4. Generic LSP / any other plugin
+
+Any plugin that accepts a system prompt can be pointed at `.mex/ROUTER.md`. The pattern:
+
+```lua
+-- Read the ROUTER once and pass it in as the system prompt
+local mex_prompt = table.concat(vim.fn.readfile(".mex/ROUTER.md"), "\n")
+```
+
+Then feed `mex_prompt` into whatever field your plugin exposes (system prompt, instructions, custom context, etc).
+
+If the plugin doesn't take a file, paste this line into its system prompt instead:
+
+```text
+Read .mex/ROUTER.md in the current working directory and follow its routing instructions.
+```
+
+That one line is enough — mex's ROUTER.md handles the rest.
+
+## Verifying your setup
+
+After any of the above, run:
+
+```bash
+mex check
+```
+
+If the scaffold is wired up correctly, mex will pick up your project state and show any drift between config files.