add El LSP skeleton — language server, VSCode extension, syntax highlighting

Implements a Language Server Protocol server for El files over stdin/stdout
JSON-RPC. Provides completions (builtins + keywords + document functions),
hover documentation, go-to-definition, and full document sync.

Also adds a VSCode extension that launches the binary as a child process and
a TextMate grammar for .el syntax highlighting.

NOTE: el-lsp.el calls __read_n(n: Int) -> String, a seed primitive not yet
in el_runtime.c. Build.sh documents the required C implementation; the seed
agent must add it before the binary will link.

tools/lsp/vscode-extension/extension.js

@@ -0,0 +1,97 @@
+// tools/lsp/vscode-extension/extension.js — El Language VSCode extension
+//
+// Launches el-lsp as a child process (stdin/stdout Language Server Protocol)
+// and connects VSCode's language client to it.
+//
+// Prerequisites:
+//   - el-lsp binary built at ../dist/el-lsp  (run tools/lsp/build.sh first)
+//   - npm install  (installs vscode-languageclient)
+//
+// Install for development:
+//   1. Open the vscode-extension/ folder in VSCode
+//   2. Press F5 — Extension Development Host launches
+//   3. Open any .el file — the LSP activates
+//
+// To package as a .vsix:
+//   npm install -g @vscode/vsce
+//   cd tools/lsp/vscode-extension && vsce package
+
+'use strict';
+
+const path = require('path');
+const { workspace, window, ExtensionContext } = require('vscode');
+const {
+    LanguageClient,
+    TransportKind,
+} = require('vscode-languageclient/node');
+
+let client;
+
+/**
+ * activate — called when a .el file is first opened.
+ * Resolves the el-lsp binary path and starts the language client.
+ *
+ * @param {ExtensionContext} context
+ */
+function activate(context) {
+    // Resolve el-lsp binary path.
+    // Default: ../dist/el-lsp relative to this extension root.
+    // Override with the "el.lspPath" workspace setting.
+    const config = workspace.getConfiguration('el');
+    const defaultLspPath = path.join(context.extensionPath, '..', 'dist', 'el-lsp');
+    const serverPath = config.get('lspPath', defaultLspPath);
+
+    // ServerOptions: run el-lsp as a child process over stdin/stdout.
+    const serverOptions = {
+        run: {
+            command: serverPath,
+            transport: TransportKind.stdio,
+        },
+        debug: {
+            command: serverPath,
+            transport: TransportKind.stdio,
+            // In debug mode you could add --verbose or redirect stderr to a
+            // log file for inspection.  For now debug == run.
+        },
+    };
+
+    // ClientOptions: handle all files with language id "el".
+    const clientOptions = {
+        documentSelector: [
+            { scheme: 'file', language: 'el' },
+            { scheme: 'untitled', language: 'el' },
+        ],
+        synchronize: {
+            // Re-send full document on save (matches textDocumentSync: 1).
+            fileEvents: workspace.createFileSystemWatcher('**/*.el'),
+        },
+        // Output channel for LSP messages (visible in Output → El Language Server).
+        outputChannelName: 'El Language Server',
+    };
+
+    client = new LanguageClient(
+        'el-language-server',
+        'El Language Server',
+        serverOptions,
+        clientOptions
+    );
+
+    // Start the client (and the server as its child process).
+    client.start();
+
+    // Notify user on activation.
+    window.showInformationMessage('El Language Server started.');
+}
+
+/**
+ * deactivate — called when the extension is unloaded.
+ * Stops the language client (sends LSP shutdown + exit).
+ */
+function deactivate() {
+    if (!client) {
+        return undefined;
+    }
+    return client.stop();
+}
+
+module.exports = { activate, deactivate };