Improve provider error messages with install suggestion

- Add "smarttools providers install" suggestion when command not found
- Add suggestion when provider exits with error containing "not found"
- Detect empty output and warn about unavailable model
- Show stderr hint when provider returns nothing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/smarttools/providers.py

@@ -174,7 +174,7 @@ def call_provider(provider_name: str, prompt: str, timeout: int = 300) -> Provid
         return ProviderResult(
             text="",
             success=False,
-            error=f"Command '{base_cmd}' not found. Is it installed and in PATH?"
+            error=f"Command '{base_cmd}' not found. Is it installed and in PATH?\n\nTo install AI providers, run: smarttools providers install"
         )
 
     try:
@@ -188,10 +188,22 @@ def call_provider(provider_name: str, prompt: str, timeout: int = 300) -> Provid
         )
 
         if result.returncode != 0:
+            error_msg = f"Provider exited with code {result.returncode}: {result.stderr}"
+            if "not found" in result.stderr.lower() or "not installed" in result.stderr.lower():
+                error_msg += "\n\nTo install AI providers, run: smarttools providers install"
             return ProviderResult(
                 text="",
                 success=False,
-                error=f"Provider exited with code {result.returncode}: {result.stderr}"
+                error=error_msg
+            )
+
+        # Warn if output is empty (provider ran but returned nothing)
+        if not result.stdout.strip():
+            stderr_hint = f" (stderr: {result.stderr.strip()})" if result.stderr.strip() else ""
+            return ProviderResult(
+                text="",
+                success=False,
+                error=f"Provider returned empty output{stderr_hint}.\n\nThis may mean the model is not available. Try a different provider or run: smarttools providers install"
             )
 
         return ProviderResult(text=result.stdout, success=True)