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.

Cargo.lock

@@ -1425,7 +1425,6 @@ dependencies = [
  "async-trait",
  "base64 0.22.1",
  "chrono",
- "const_format",
  "futures-util",
  "g3-computer-control",
  "g3-config",