feat(vscode-desktop): add extensions and settings pre-installation

Description

Closes #207

Adds the ability to pre-install VS Code extensions and apply machine-level settings on the remote host when a Coder workspace starts. Extensions are ready when the user opens VS Code Desktop for the first time — no manual installation needed.

Type of Change

• Feature/enhancement

Module Information

Path: registry/coder/modules/vscode-desktop
New version: v1.3.0
Breaking change: [ ] Yes [x] No

What Changed

New Variables

How It Works

A coder_script resource (install-extensions.sh) runs on workspace start when extensions or settings are specified:

1. Settings — Written to ~/.vscode-server/data/Machine/settings.json. Merges with existing settings using jq or python3 (with overwrite fallback).

2. Extensions — Installed to ~/.vscode-server/extensions/:

   • First checks for an existing VS Code CLI (from a previous Desktop connection or the vscode-web module)
   • If no CLI found, downloads VS Code Server temporarily
   • Installs each extension with --install-extension --force

3. No-op when unused — The script resource is not created when both extensions and settings are empty (count = 0).

Architecture Decision

All new functionality is contained in the vscode-desktop wrapper module — no changes to vscode-desktop-core. This is intentional and addresses the "let's solve this systematically for all VS Code-based IDEs" point raised in #207:

• Each IDE stores extensions in a different server directory: VS Code Desktop uses ~/.vscode-server/, Cursor uses ~/.cursor-server/, Windsurf uses ~/.windsurf-server/, etc. A single core-level implementation cannot cleanly handle these divergent paths.
• This PR follows the exact same per-module pattern already shipped in the vscode-web and code-server modules (each installs extensions via --install-extension and merges machine settings in its own run.sh). Consistency with the existing, merged approach.
• The extensions_dir variable is exposed (per maintainer request in Feature: Pre-Install VS Code extensions in desktop VS Code module #218) so the same script can be reused by Cursor/Windsurf/Kiro wrappers in follow-up PRs simply by pointing at their server directory.

This keeps the shared core module stable while resolving #207 directly.

Addressing #218's review feedback

The previous attempt (#218, closed) was blocked because it assumed the VS Code CLI was already present. This implementation:

• Detects an existing VS Code CLI (from a prior connection or the vscode-web module) and reuses it
• Falls back to downloading VS Code Server when no CLI is found — so extension installation works on a fresh workspace

Usage

module "vscode" {
  count    = data.coder_workspace.me.start_count
  source   = "registry.coder.com/coder/vscode-desktop/coder"
  version  = "1.3.0"
  agent_id = coder_agent.main.id
  folder   = "/home/coder/project"
  extensions = [
    "ms-python.python",
    "esbenp.prettier-vscode",
    "dbaeumer.vscode-eslint",
  ]
  settings = {
    "editor.fontSize"    = 14
    "editor.tabSize"     = 2
    "editor.formatOnSave" = true
  }
}

Testing & Validation

• Tests pass (bun test)
• No extensions/settings → no script created (zero overhead)
• Extensions specified → script includes extension IDs
• Settings specified → base64-encoded and decoded correctly
• Settings written to correct machine settings path
• Settings merged with existing (jq deep merge tested)
• Backward compatible — all existing variables and behavior unchanged