dx: add .devcontainer/ for one-click contributor setup

.devcontainer/README.md

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+# Development Container
+
+This folder contains the [Dev Container](https://containers.dev/) configuration for
+contributing to **AI Engineering Coach**. It provides a one-click, reproducible
+development environment for the VS Code extension.
+
+## What you get
+
+- Node.js 22 (LTS) on Debian Bookworm
+- `npm install` runs automatically after the container is created
+- GitHub CLI (`gh`) preinstalled
+- Recommended VS Code extensions preinstalled:
+  - ESLint (`dbaeumer.vscode-eslint`)
+  - TypeScript Next (`ms-vscode.vscode-typescript-next`)
+  - Vitest Explorer (`vitest.explorer`)
+  - Code Spell Checker (`streetsidesoftware.code-spell-checker`)
+  - Playwright Test (`ms-playwright.playwright`)
+
+## How to use it
+
+### GitHub Codespaces
+Open this repository on GitHub and choose **Code → Codespaces → Create codespace
+on main**. The container will build and dependencies will install automatically.
+
+### VS Code (Dev Containers extension)
+1. Install the [Dev Containers](https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers)
+   extension.
+2. Open the cloned repository in VS Code.
+3. Run **Dev Containers: Reopen in Container** from the Command Palette.
+
+Once the container is ready, run `npm run check` to verify your setup (typecheck,
+lint, spellcheck, knip, and tests).

.devcontainer/devcontainer.json

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+{
+  "name": "AI Engineering Coach",
+  "image": "mcr.microsoft.com/devcontainers/typescript-node:22-bookworm",
+  "features": {
+    "ghcr.io/devcontainers/features/github-cli:1": {}
+  },
+  "postCreateCommand": "npm install",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "dbaeumer.vscode-eslint",
+        "ms-vscode.vscode-typescript-next",
+        "vitest.explorer",
+        "streetsidesoftware.code-spell-checker",
+        "ms-playwright.playwright"
+      ],
+      "settings": {
+        "editor.formatOnSave": true,
+        "editor.codeActionsOnSave": {
+          "source.fixAll.eslint": "explicit"
+        },
+        "typescript.tsdk": "node_modules/typescript/lib"
+      }
+    }
+  },
+  "remoteUser": "node"
+}