Add quickstart guide for skill creators

Adds a tutorial-style quickstart guide that walks the reader through
creating their first Agent Skill — a `roll-dice` skill that teaches an
agent to roll dice using true system randomness. The guide covers
creating the `SKILL.md` file, verifying discovery via `/skills` in VS
Code, testing with a "Roll a d20" prompt, and a brief explanation of the
discovery/activation/execution lifecycle. Includes both bash and
PowerShell command variants, and a note about model variation in
tool-use reliability.

Also adds the quickstart as the first page under "For skill creators" in
`docs.json` navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/docs.json

@@ -22,6 +22,7 @@
       {
         "group": "For skill creators",
         "pages": [
+          "skill-creation/quickstart",
           "skill-creation/best-practices",
           "skill-creation/optimizing-descriptions",
           "skill-creation/evaluating-skills",

docs/skill-creation/quickstart.mdx

@@ -0,0 +1,80 @@
+---
+title: "Quickstart"
+description: "Create your first Agent Skill and see it work in VS Code."
+---
+
+In this tutorial, you'll create a skill that gives an agent the capability to roll dice using true randomness.
+
+## Prerequisites
+
+- [VS Code](https://code.visualstudio.com/) with [GitHub Copilot](https://marketplace.visualstudio.com/items?itemName=GitHub.copilot)
+
+<Note>
+This tutorial uses VS Code, but Agent Skills are an open format. The same skill works in any compatible agent, including Claude Code and OpenAI Codex.
+</Note>
+
+## Create the skill
+
+A skill is a folder containing a `SKILL.md` file. VS Code looks for skills in `.agents/skills/` by default. Create `.agents/skills/roll-dice/SKILL.md` in your project:
+
+````markdown .agents/skills/roll-dice/SKILL.md
+---
+name: roll-dice
+description: Roll dice with true randomness. Use when asked to roll a die (d6, d20, etc.), roll dice, or generate a random dice roll.
+---
+
+To roll a die, use the following command that generates a random number from 1
+to the given number of sides:
+
+```bash
+shuf -i 1-<sides> -n 1
+```
+
+```powershell
+Get-Random -Minimum 1 -Maximum (<sides> + 1)
+```
+
+Replace `<sides>` with the number of sides on the die (e.g., 6 for a standard
+die, 20 for a d20).
+````
+
+That's it — one file, under 20 lines. Here's what each part does:
+
+- **`name`** — A short identifier for the skill. Must match the folder name.
+- **`description`** — Tells the agent when to use this skill. This is how the agent decides whether to activate it.
+- **The body** — Instructions the agent follows when the skill activates. Here, the agent is instructed to generate a random number using a terminal command, substituting the number of sides from the user's request.
+
+## Try it out
+
+1. Open your project in VS Code.
+2. Open the Copilot Chat panel.
+3. Select **Agent** mode from the mode dropdown at the bottom of the chat panel.
+4. Type `/skills` to confirm that `roll-dice` appears in the list. If it doesn't, check that the file is at `.agents/skills/roll-dice/SKILL.md` relative to your project root.
+5. Ask: **"Roll a d20"**
+
+The agent should activate the `roll-dice` skill. It may ask for permission to run a terminal command — allow it. It will run the command and return a random number between 1 and 20.
+
+<Note>
+Tool-use reliability varies across models — some follow skill instructions and run commands consistently, while others may attempt to answer on their own. If the agent responds without running a terminal command, try selecting a different model from the model dropdown.
+</Note>
+
+## How it works
+
+Here's what happened behind the scenes:
+
+1. **Discovery** — When the chat session started, the agent scanned default skill directories and found your skill. It read only the `name` and `description`, just enough to know when the skill might be relevant.
+
+2. **Activation** — When you asked about rolling dice, the agent matched your question to the skill's description and loaded the full `SKILL.md` body into context.
+
+3. **Execution** — The agent followed the instructions in the body, adapting the terminal command to the number of sides in your request.
+
+This process uses **progressive disclosure** to let the agent access many skills without loading all their instructions up front. For more detail, see [How skills work](/what-are-skills#how-skills-work).
+
+## Next steps
+
+You've created a working Agent Skill. From here:
+
+- **[Best practices](/skill-creation/best-practices)** — How to write skills that are well-scoped and effective.
+- **[Optimizing skill descriptions](/skill-creation/optimizing-descriptions)** — Test and improve your skill's description so it activates on the right prompts.
+- **[Specification](/specification)** — The complete format reference for `SKILL.md` files.
+- **[Example skills](https://github.com/anthropics/skills)** — Browse real-world skills on GitHub.