Simplify system prompt: remove coding style and parallel tool call sections

- Remove IMPORTANT FOR CODING section (~1,500 chars of coding guidelines)
- Remove <use_parallel_tool_calls> block (~500 chars)
- Remove unused const_format dependency from g3-core
- Simplify get_system_prompt_for_native() to just return base prompt
- Response Guidelines now cleanly ends the static prompt

Prompt reduced from ~8,500 to ~6,500 characters.

crates/g3-core/Cargo.toml

@@ -43,7 +43,6 @@ tree-sitter-scheme = "0.24"
 streaming-iterator = "0.1"
 walkdir = "2.4"
 
-const_format = "0.2"
 base64 = "0.22.1"
 
 [dev-dependencies]