docs: add Vim/Neovim setup guide

[mvanhorn]

What

Adds docs/vim-neovim.md, a standalone Neovim setup guide for mex, and links it from the Multi-Tool Compatibility section of README.md.

The guide covers four setups:

1. Claude Code in Neovim's terminal (:term claude + existing CLAUDE.md)
2. Avante.nvim pointed at .mex/ROUTER.md
3. Copilot.vim / copilot.lua via .github/copilot-instructions.md (setup.sh option 4)
4. Generic LSP / any plugin: either read .mex/ROUTER.md into the system prompt, or paste a one-line instruction telling the plugin to read it from the cwd

Why

Closes #14.

Vim/Neovim users can use mex today because the scaffold is tool-agnostic, but the README's Multi-Tool Compatibility table only names the configs the other editors consume. There's no guide for wiring up Neovim's common AI plugins, so users were left to improvise. The new guide lists each working path and the exact setup.sh option (or plugin config snippet) that produces it.

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. Read docs/vim-neovim.md and follow one of the sections.
2. For section 1 (Claude Code in terminal): open the repo in Neovim, run :term claude, and confirm Claude picks up CLAUDE.md automatically. This was tested end-to-end.
3. For sections 2-4: each section references the exact file or option (.mex/ROUTER.md, setup.sh option 4, option 8) that already exists in this repo.

Checklist

• Scope is limited to docs/vim-neovim.md (new) and the single README paragraph linking it.
• No code, test, or config changes.
• Followed the existing README section ordering; the link sits directly under the Multi-Tool Compatibility paragraph.

Fixes #14

This contribution was developed with AI assistance (Codex).

[theDakshJaitly]

LGTM — thanks for the clear, scoped guide. Pushed one small fix on top: vim.fn.readfile() returns a list of lines, so the generic-plugin snippet needed table.concat(..., "\n") to produce a string that plugins can feed into system_prompt.