fix: convert output to default to the source root

docs/convert.md

@@ -55,7 +55,7 @@ This will output something like:
 <span class="muted">│</span>
 <span class="muted">└</span>  1 notebook converted; 2 files written</code></pre>
 
-The `convert` command generates files in the current working directory. The command above generates two files: <code>zoomable-sunburst.md</code>, a Markdown file representing the converted notebook; and <code>flare-2.json</code>, an attached JSON file. You can change the output directory using the <code>--output</code> command-line flag.
+The `convert` command generates files in the source root of the current project by default (typically `src`); you can change the output directory using the `--output` command-line flag. The command above generates two files: `zoomable-sunburst.md`, a Markdown file representing the converted notebook; and `flare-2.json`, an attached JSON file.
 
 Due to differences between Observable Framework and Observable notebooks, the `convert` command typically won’t produce a working Markdown page out of the box; you’ll often need to make further edits to the generated Markdown. We describe these differences below, along with examples of manual conversion.
 

src/bin/observable.ts

@@ -1,4 +1,5 @@
 #!/usr/bin/env node
+import {join} from "node:path/posix";
 import type {ParseArgsConfig} from "node:util";
 import {parseArgs} from "node:util";
 import * as clack from "@clack/prompts";
@@ -199,12 +200,27 @@ try {
     case "convert": {
       const {
         positionals,
-        values: {output, force}
+        values: {config, root, output: out, force}
       } = helpArgs(command, {
-        options: {output: {type: "string", default: "."}, force: {type: "boolean", short: "f"}},
+        options: {
+          output: {
+            type: "string",
+            short: "o",
+            description: "Output directory (defaults to the source root)"
+          },
+          force: {
+            type: "boolean",
+            short: "f",
+            description: "If true, overwrite existing resources"
+          },
+          ...CONFIG_OPTION
+        },
         allowPositionals: true
       });
-      await import("../convert.js").then((convert) => convert.convert(positionals, {output: output!, force}));
+      // The --output command-line option is relative to the cwd, but the root
+      // config option (typically "src") is relative to the project root.
+      const output = out ?? join(root ?? ".", (await readConfig(config, root)).root);
+      await import("../convert.js").then((convert) => convert.convert(positionals, {output, force}));
       break;
     }
     default: {