feat: Add web-only fallback mode with API key promo

- Skill now works without any API keys using WebSearch fallback
- Shows promo banner marketing Reddit/X data when keys are missing
- Partial mode (one key) shows shorter tip for the missing source
- Updated SKILL.md to document three modes: Full, Partial, Web-Only
- Added get_missing_keys() to env.py for promo logic
- Added show_promo(), start_web_only(), show_web_only_complete() to ui.py
- Updated render_compact() to include inline promo for web-only mode

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/lib/env.py

@@ -72,6 +72,24 @@ def get_available_sources(config: Dict[str, Any]) -> str:
         return 'web'  # Fallback: WebSearch only (no API keys needed)
 
 
+def get_missing_keys(config: Dict[str, Any]) -> str:
+    """Determine which API keys are missing.
+
+    Returns: 'both', 'reddit', 'x', or 'none'
+    """
+    has_openai = bool(config.get('OPENAI_API_KEY'))
+    has_xai = bool(config.get('XAI_API_KEY'))
+
+    if has_openai and has_xai:
+        return 'none'
+    elif has_openai:
+        return 'x'  # Missing xAI key
+    elif has_xai:
+        return 'reddit'  # Missing OpenAI key
+    else:
+        return 'both'  # Missing both keys
+
+
 def validate_sources(requested: str, available: str, include_web: bool = False) -> tuple[str, Optional[str]]:
     """Validate requested sources against available keys.