docs: add mcp and skill session config (#106)

docs/attachments.mdx

@@ -0,0 +1,87 @@
+---
+title: "Attachments"
+description: "Upload files into the sandbox and attach them to prompts."
+sidebarTitle: "Attachments"
+icon: "paperclip"
+---
+
+Use the filesystem API to upload files, then reference them as attachments when sending prompts.
+
+<Steps>
+  <Step title="Upload a file">
+    <CodeGroup>
+    ```ts TypeScript
+    import { SandboxAgent } from "sandbox-agent";
+    import fs from "node:fs";
+
+    const client = await SandboxAgent.connect({
+      baseUrl: "http://127.0.0.1:2468",
+      token: process.env.SANDBOX_TOKEN,
+    });
+
+    const buffer = await fs.promises.readFile("./data.csv");
+
+    const upload = await client.writeFsFile(
+      { path: "./uploads/data.csv", sessionId: "my-session" },
+      buffer,
+    );
+
+    console.log(upload.path);
+    ```
+
+    ```bash cURL
+    curl -X PUT "http://127.0.0.1:2468/v1/fs/file?path=./uploads/data.csv&sessionId=my-session" \
+      -H "Authorization: Bearer $SANDBOX_TOKEN" \
+      --data-binary @./data.csv
+    ```
+    </CodeGroup>
+
+    The response returns the absolute path that you should use for attachments.
+  </Step>
+
+  <Step title="Attach the file in a prompt">
+    <CodeGroup>
+    ```ts TypeScript
+    import { SandboxAgent } from "sandbox-agent";
+
+    const client = await SandboxAgent.connect({
+      baseUrl: "http://127.0.0.1:2468",
+      token: process.env.SANDBOX_TOKEN,
+    });
+
+    await client.postMessage("my-session", {
+      message: "Please analyze the attached CSV.",
+      attachments: [
+        {
+          path: "/home/sandbox/uploads/data.csv",
+          mime: "text/csv",
+          filename: "data.csv",
+        },
+      ],
+    });
+    ```
+
+    ```bash cURL
+    curl -X POST "http://127.0.0.1:2468/v1/sessions/my-session/messages" \
+      -H "Authorization: Bearer $SANDBOX_TOKEN" \
+      -H "Content-Type: application/json" \
+      -d '{
+        "message": "Please analyze the attached CSV.",
+        "attachments": [
+          {
+            "path": "/home/sandbox/uploads/data.csv",
+            "mime": "text/csv",
+            "filename": "data.csv"
+          }
+        ]
+      }'
+    ```
+    </CodeGroup>
+  </Step>
+</Steps>
+
+## Notes
+
+- Use absolute paths from the upload response to avoid ambiguity.
+- If `mime` is omitted, the server defaults to `application/octet-stream`.
+- OpenCode receives file parts directly; other agents will see the attachment paths appended to the prompt.