Optimize native system prompt - 48% size reduction

Removed redundant and vague content from prompts/system/native.md:
- Simplified intro from 17 lines to 3 lines
- Reduced Code Search section to one line
- Removed duplicate Plan Mode example (kept one)
- Removed Action Envelope section (rarely used correctly)
- Removed verbose Memory Format details (tool description covers it)
- Removed Response Guidelines (obvious to modern LLMs)

Size: 8,620 chars -> 4,498 chars

Also updated:
- G3_IDENTITY_LINE constant for agent mode compatibility
- Test assertions to check for new prompt markers
- System prompt validation to use new marker string

crates/g3-core/src/lib.rs

@@ -438,12 +438,12 @@ impl<W: UiWriter> Agent<W> {
 
         // Check for system prompt markers that are present in both standard and agent mode
         // Check for system prompt markers that are present in both native and non-native prompts
-        // Both prompts contain "You have access to tools" as a common marker
+        // Both prompts contain "Use tools to accomplish tasks" as a common marker
         let has_tool_instructions = first_message
             .content
-            .contains("You have access to tools");
+            .contains("Use tools to accomplish tasks");
         if !has_tool_instructions {
-            panic!("FATAL: First system message does not contain the system prompt marker 'You have access to tools'. This likely means the README was added before the system prompt.");
+            panic!("FATAL: First system message does not contain the system prompt marker 'Use tools to accomplish tasks'. This likely means the README was added before the system prompt.");
         }
     }