fix: remove allow_multiple_tool_calls config and simplify tool execution flow

This fixes a bug where the agent would stop responding abruptly without
calling final_output. The root cause was the allow_multiple_tool_calls
config option (default: false) which caused the agent to break out of
the streaming loop mid-stream after executing the first tool, losing
any subsequent content.

Changes:
- Remove allow_multiple_tool_calls config option entirely
- Always process all tool calls without breaking mid-stream
- Simplify system prompt generation (no longer needs boolean param)
- Let the stream complete fully before continuing to next iteration
- Change find_last_tool_call_start to find_first_tool_call_start
- Remove parser.reset() call on duplicate detection

Benefits:
- Simpler logic with less conditional branching
- No lost content after tool calls
- Consistent behavior for all users
- Reduced config complexity

crates/g3-cli/src/lib.rs

@@ -759,7 +759,8 @@ async fn run_agent_mode(
     let config = g3_config::Config::load(config_path)?;
     
     // Generate the combined system prompt (agent prompt + tool instructions)
-    let system_prompt = get_agent_system_prompt(&agent_prompt, config.agent.allow_multiple_tool_calls);
+    // Note: allow_multiple_tool_calls parameter is deprecated but kept for API compatibility
+    let system_prompt = get_agent_system_prompt(&agent_prompt, true);
     
     // Read README if present
     let readme_content = std::fs::read_to_string(workspace_dir.join("README.md")).ok();