feat: Dual-OS Neovim MCP Integration

- Created linux-nvim and win-nvim MCP servers for deep Neovim Msgpack-RPC integration.
- Implemented tools: nvim_goto_line, nvim_get_active_buffer, nvim_get_cursor, nvim_get_visual_selection, nvim_set_diagnostics.
- Added gemini-integration.lua for active UI socket discovery.
- Updated build.cmd to cross-compile for both Windows (gnu) and Linux (musl).
- Updated README, design.md, and instructions.md with architecture and tool capabilities.

Cargo.toml

@@ -1,6 +1,8 @@
 [workspace]
 members = [
     "server",
-    "stub"
+    "stub",
+    "win-nvim",
+    "linux-nvim"
 ]
 resolver = "2"