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  + repo no longer actively maintained)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.py

@@ -255,7 +255,7 @@ def run_spec_creation(project_dir: Path) -> bool:
             print(f"Please ensure app_spec.txt exists in: {get_project_prompts_dir(project_dir)}")
             # If failed with non-zero exit and no spec, might be auth issue
             if result.returncode != 0:
-                print("\nIf you're having authentication issues, try running: claude login")
+                print("\nIf you're having authentication issues, set ANTHROPIC_API_KEY or try: claude login")
             return False
 
     except FileNotFoundError:
@@ -416,7 +416,7 @@ def run_agent(project_name: str, project_dir: Path) -> None:
                 print(f"\nAgent error:\n{stderr_output.strip()}")
                 # Still hint about auth if exit was unexpected
                 if "error" in stderr_output.lower() or "exception" in stderr_output.lower():
-                    print("\nIf this is an authentication issue, try running: claude login")
+                    print("\nIf this is an authentication issue, set ANTHROPIC_API_KEY or try: claude login")
 
     except KeyboardInterrupt:
         print("\n\nAgent interrupted. Run again to resume.")