feat: add auth to publish command and document authentication

- Publish command now requires login and sends auth token
- Added Authentication section to README with login/register docs
- Uses L2 server for authentication when publishing runs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

artdag.py

@@ -385,22 +385,34 @@ def assets():
 @cli.command()
 @click.argument("run_id")
 @click.argument("output_name")
-@click.option("--l2", envvar="ARTDAG_L2", default="http://localhost:8200",
-              help="L2 server URL")
-def publish(run_id, output_name, l2):
-    """Publish an L1 run to L2 (register ownership).
+def publish(run_id, output_name):
+    """Publish an L1 run to L2 (register ownership). Requires login.
 
     RUN_ID: The L1 run ID to publish
     OUTPUT_NAME: Name for the registered asset
     """
-    # Post to L2 server
-    resp = requests.post(
-        f"{l2}/registry/record-run",
-        json={"run_id": run_id, "output_name": output_name}
-    )
-    resp.raise_for_status()
-    result = resp.json()
+    # Check auth
+    token_data = load_token()
+    if not token_data.get("access_token"):
+        click.echo("Not logged in. Please run: artdag login <username>", err=True)
+        sys.exit(1)
 
+    # Post to L2 server with auth
+    try:
+        resp = requests.post(
+            f"{get_l2_server()}/registry/record-run",
+            json={"run_id": run_id, "output_name": output_name},
+            headers={"Authorization": f"Bearer {token_data['access_token']}"}
+        )
+        if resp.status_code == 401:
+            click.echo("Authentication failed. Please login again.", err=True)
+            sys.exit(1)
+        resp.raise_for_status()
+    except requests.RequestException as e:
+        click.echo(f"Failed to publish: {e}", err=True)
+        sys.exit(1)
+
+    result = resp.json()
     click.echo(f"Published to L2!")
     click.echo(f"Asset: {result['asset']['name']}")
     click.echo(f"Hash: {result['asset']['content_hash']}")