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.

gemini-integration.lua

@@ -0,0 +1,26 @@
+-- gemini-integration.lua
+-- Registers the active Neovim instance for the Antigravity MCP Server
+vim.api.nvim_create_autocmd({"VimEnter", "FocusGained"}, {
+    callback = function()
+        -- Only register if Neovim has an active UI attached (ignore headless/embedded instances)
+        if #vim.api.nvim_list_uis() > 0 then
+            local server_name = vim.v.servername
+            if server_name then
+                local home = os.getenv("HOME")
+                if not home then
+                    home = os.getenv("USERPROFILE")
+                end
+                if home then
+                    os.execute("mkdir -p " .. home .. "/.gemini")
+                    local path = home .. "/.gemini/active_nvim.txt"
+                    local f = io.open(path, "w")
+                    if f then
+                        f:write(server_name)
+                        f:close()
+                    end
+                end
+            end
+        end
+    end,
+    desc = "Register active Neovim instance for Gemini CLI",
+})