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>

README.md

@@ -18,6 +18,46 @@ export ARTDAG_SERVER=http://localhost:8100
 ./artdag.py --server http://localhost:8100 <command>
 ```
 
+## Authentication
+
+The client authenticates against an L2 server for commands that require login (e.g., `run`, `publish`).
+
+```bash
+# Set L2 server URL (default: http://localhost:8200)
+export ARTDAG_L2=https://artdag.rose-ash.com
+
+# Or pass with every command
+./artdag.py --l2 https://artdag.rose-ash.com <command>
+```
+
+### Login
+```bash
+./artdag.py login <username>
+# You'll be prompted for your password
+
+# Or specify password with -p flag (will prompt)
+./artdag.py login <username> -p
+```
+
+### Register
+```bash
+./artdag.py register <username>
+# You'll be prompted to enter and confirm your password
+
+# Optionally include email
+./artdag.py register <username> --email user@example.com
+```
+
+### Check Current User
+```bash
+./artdag.py whoami
+```
+
+### Logout
+```bash
+./artdag.py logout
+```
+
 ## Commands
 
 ### Server Info