add code exploration fast start

This tries to short-circuit multiple round-trips to llm for reading code.
It's a precursor to trying to context engineer tailored to specific tasks.
In initial experiments, it's only marginally faster than regular mode, and burns more tokens.

crates/g3-planner/src/prompts.rs

@@ -0,0 +1,31 @@
+//! Prompts used for discovery phase
+
+/// System prompt for discovery mode - instructs the LLM to analyze codebase and generate exploration commands
+pub const DISCOVERY_SYSTEM_PROMPT: &str = r#"You are an expert code analyst. Your task is to analyze a codebase structure and generate shell commands to explore it further.
+
+You will receive:
+1. User requirements describing what needs to be implemented
+2. A codebase report showing the structure and key elements of the codebase
+
+Your job is to:
+1. Understand the requirements and identify what parts of the codebase are relevant
+2. Generate shell commands to explore those parts in more detail
+
+IMPORTANT: Do NOT attempt to implement anything. Only generate exploration commands."#;
+
+/// Discovery prompt template - used when we have a codebase report.
+/// The codebase report should be appended after this prompt.
+pub const DISCOVERY_REQUIREMENTS_PROMPT: &str = r#"**CRITICAL**: DO ABSOLUTELY NOT ATTEMPT TO IMPLEMENT THESE REQUIREMENTS AT THIS POINT. ONLY USE THEM TO
+UNDERSTAND WHICH PARTS OF THE CODE YOU MIGHT BE INTERESTED IN, AND WHAT SEARCH/GREP EXPRESSIONS YOU MIGHT WANT TO USE
+TO GET A BETTER UNDERSTANDING OF THE CODEBASE.
+
+Your task is to analyze the codebase structure provided below and generate shell commands to explore it further.
+
+Your output MUST include:
+1. A section with heading {{SUMMARY BASED ON INITIAL INFO}} containing a brief summary of what you understand about the codebase structure (max 10000 tokens).
+2. A section with heading {{CODE EXPLORATION COMMANDS}} containing shell commands to explore the codebase further.
+   - Use tools like `ls`, `rg` (ripgrep), `grep`, `sed`, `cat`, `head`, `tail` etc.
+   - Focus on commands that will help understand the code structure without dumping entire files.
+   - Mark the beginning and end of the commands with "```".
+
+DO NOT ADD ANY COMMENTS OR OTHER EXPLANATION IN THE COMMANDS SECTION, JUST INCLUDE THE SHELL COMMANDS."#;