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  + repo no longer actively maintained)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.sh

@@ -25,11 +25,12 @@ echo "[OK] Claude CLI found"
 # verify the CLI is installed and remind the user to login if needed
 if [ -d "$HOME/.claude" ]; then
     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 ""
     read -p "Press Enter to continue anyway, or Ctrl+C to exit..."
 fi