Add Sprites docs page and mark Slack/exe.dev as in progress

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/administration/sandboxing.mdx

@@ -5,7 +5,7 @@ description: "Sandboxing workflow execution"
 
 Sandboxes isolate agent execution from the host machine. When an agent runs a shell command, edits a file, or searches code, it does so inside a sandbox — preventing unintended side effects on the host and providing a reproducible environment for each run.
 
-Arc supports four sandbox providers: `local` (no isolation), `docker` (container-level), `daytona` (cloud VM), and `exe` (cloud VM via [exe.dev](https://exe.dev)). See [Environments](/execution/environments) for full provider-specific configuration.
+Arc supports five sandbox providers: `local` (no isolation), `docker` (container-level), `daytona` (cloud VM), `exe` (cloud VM via [exe.dev](https://exe.dev)), and `sprites` (persistent VM via [Sprites](https://sprites.dev), in progress). See [Environments](/execution/environments) for full provider-specific configuration.
 
 ## Network access control
 

docs/docs.json

@@ -93,6 +93,7 @@
               "integrations/daytona",
               "integrations/slack",
               "integrations/exe-dev",
+              "integrations/sprites",
               "integrations/brave-search"
             ]
           },

docs/integrations/exe-dev.mdx

@@ -4,7 +4,7 @@ description: "Run Arc workflows in fast ephemeral VMs from exe.dev"
 ---
 
 <Warning>
-The exe.dev sandbox provider is in **beta** and not yet generally available. This page describes the planned integration. We'll update it when exe.dev support ships.
+The exe.dev sandbox provider is **in progress** and not yet available for use. This page describes the planned integration. We'll update it when exe.dev support ships.
 </Warning>
 
 [exe.dev](https://exe.dev) provides fast ephemeral cloud VMs for Arc workflows. Each run gets a fresh VM with full machine-level isolation, connected via SSH.

docs/integrations/slack.mdx

@@ -3,6 +3,10 @@ title: "Slack"
 description: "Answer human-in-the-loop questions directly from Slack"
 ---
 
+<Warning>
+The Slack integration is **in progress** and not yet available for use. This page describes the planned integration. We'll update it when Slack support ships.
+</Warning>
+
 Arc's Slack integration lets your team answer [human-in-the-loop](/workflows/human-in-the-loop) questions without leaving Slack. When a workflow reaches a human gate, Arc posts an interactive message to a Slack channel with buttons for each option. Team members click a button (or reply in a thread for freeform input), and the workflow continues.
 
 The integration uses Slack's [Socket Mode](https://api.slack.com/apis/socket-mode), so no public URL or webhook endpoint is required — Arc connects outbound over a WebSocket.

docs/integrations/sprites.mdx

@@ -0,0 +1,64 @@
+---
+title: "Sprites"
+description: "Run Arc workflows in persistent Sprites VMs from Fly.io"
+---
+
+<Warning>
+The Sprites sandbox provider is **in progress** and not yet available for use. This page describes the planned integration. We'll update it when Sprites support ships.
+</Warning>
+
+[Sprites](https://sprites.dev) (by Fly.io) provides persistent, hardware-isolated Linux microVMs for Arc workflows. Unlike ephemeral providers, Sprites VMs persist across runs with auto-sleep/wake — so dependencies stay installed and caches stay warm.
+
+## How it works
+
+- Each run creates (or reuses) a Sprite VM via the `sprite` CLI
+- Agent tool calls (shell commands, file edits, grep, glob) execute via `sprite exec` inside the VM
+- The working directory is `/home/sprite`
+- Each Sprite has a preview URL (e.g. `https://<name>-<org>.sprites.app/`)
+- VMs auto-sleep after inactivity and wake on the next command
+- Pre-installed toolchains include ripgrep
+
+## Configuration
+
+```bash
+arc run start workflow.dot --sandbox sprites
+```
+
+```toml title="run.toml"
+[sandbox]
+provider = "sprites"
+```
+
+A full configuration example with all Sprites-specific options:
+
+```toml title="run.toml"
+[sandbox]
+provider = "sprites"
+
+[sandbox.sprites]
+org = "my-org"
+url_auth = "public"
+sprite_name = "my-existing-sprite"
+```
+
+| Option | Description |
+|---|---|
+| `org` | Sprites organization name (passed as `--org` to the CLI) |
+| `url_auth` | Preview URL auth mode: `"public"` or `"sprite"` (default) |
+| `sprite_name` | Reuse an existing Sprite instead of creating a new one. Skips create and destroy — useful for development |
+
+## Prerequisites
+
+- The `sprite` CLI installed and authenticated (`sprite login`)
+- A `SPRITE_TOKEN` environment variable (or interactive login)
+
+## Reusing Sprites
+
+Set `sprite_name` to reuse an existing Sprite across runs. When set, Arc skips creation and destruction — the Sprite persists between runs with its filesystem intact:
+
+```toml title="run.toml"
+[sandbox.sprites]
+sprite_name = "my-dev-sprite"
+```
+
+This is useful for development workflows where you want to keep installed dependencies and project state between runs.