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  + repo no longer actively maintained)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_ui.sh

@@ -16,16 +16,16 @@ if ! command -v claude &> /dev/null; then
     echo "    The agent requires Claude CLI to work."
     echo "    Install it from: https://claude.ai/download"
     echo ""
-    echo "    After installing, run: claude login"
+    echo "    After installing, set ANTHROPIC_API_KEY or run: claude login"
     echo ""
 else
     echo "[OK] Claude CLI found"
     # Note: Claude CLI no longer stores credentials in ~/.claude/.credentials.json
     # We can't reliably check auth status without making an API call
     if [ -d "$HOME/.claude" ]; then
-        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 - run 'claude login' first"
+        echo "[!] Claude CLI not configured - set ANTHROPIC_API_KEY or run 'claude login'"
     fi
 fi
 echo ""