feat: add auth policy disclaimer and repo maintenance notice

Add prominent warnings about Anthropic's Agent SDK policy regarding
subscription-based authentication for third-party agents. Users are
now advised to use API keys instead of `claude login` to avoid
potential account suspension.

Changes:
- README: Add WARNING and NOTE admonition boxes at top (auth policy
  + repo no longer actively maintained)
- README: Flip auth recommendation to API key first, subscription second
- SettingsModal: Add amber warning Alert when Claude provider is selected
- auth.py: Update CLI/server help messages to recommend API key as Option 1
- Start scripts (start.sh, start.bat, start_ui.sh): Mention ANTHROPIC_API_KEY
  alongside claude login in all auth hints
- start.py, autonomous_agent_demo.py: Update help text references

No functionality removed — subscription auth still works, warnings are
informational only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

start.bat

@@ -29,11 +29,12 @@ REM verify the CLI is installed and remind the user to login if needed
 set "CLAUDE_DIR=%USERPROFILE%\.claude"
 if exist "%CLAUDE_DIR%\" (
     echo [OK] Claude CLI directory found
-    echo      ^(If you're not logged in, run: claude login^)
+    echo      ^(Set ANTHROPIC_API_KEY or run: claude login^)
 ) else (
     echo [!] Claude CLI not configured
     echo.
-    echo Please run 'claude login' to authenticate before continuing.
+    echo Please set ANTHROPIC_API_KEY or run 'claude login' to authenticate.
+    echo Note: API key auth is recommended. See README for details.
     echo.
     pause
 )