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/tests/planner_test.rs

@@ -0,0 +1,103 @@
+//! Integration tests for g3-planner
+
+use g3_planner::{create_tool_message, explore_codebase, extract_shell_commands};
+use g3_providers::MessageRole;
+
+#[test]
+fn test_create_tool_message_format() {
+    let msg = create_tool_message("shell", "ls -la");
+
+    assert!(matches!(msg.role, MessageRole::Assistant));
+
+    let parsed: serde_json::Value = serde_json::from_str(&msg.content).unwrap();
+    assert_eq!(parsed["tool"], "shell");
+    assert_eq!(parsed["args"]["command"], "ls -la");
+}
+
+#[test]
+fn test_explore_codebase_returns_report() {
+    // Test with current directory (should find Rust files in g3 project)
+    let report = explore_codebase(".");
+
+    // Should return a non-empty report
+    assert!(!report.is_empty(), "Report should not be empty");
+
+    // Should contain the codebase analysis header
+    assert!(
+        report.contains("CODEBASE ANALYSIS") || report.contains("No recognized"),
+        "Report should have analysis header or indicate no languages found"
+    );
+}
+
+#[test]
+fn test_extract_shell_commands_basic() {
+    let response = r#"
+Some text here.
+
+{{CODE EXPLORATION COMMANDS}}
+
+```bash
+ls -la
+cat README.md
+rg --files -g '*.rs'
+```
+
+More text.
+"#;
+
+    let commands = extract_shell_commands(response);
+    assert_eq!(commands.len(), 3);
+    assert_eq!(commands[0], "ls -la");
+    assert_eq!(commands[1], "cat README.md");
+    assert_eq!(commands[2], "rg --files -g '*.rs'");
+}
+
+#[test]
+fn test_extract_shell_commands_with_comments() {
+    let response = r#"
+{{CODE EXPLORATION COMMANDS}}
+
+```
+# This is a comment
+ls -la
+# Another comment
+cat file.txt
+```
+"#;
+
+    let commands = extract_shell_commands(response);
+    assert_eq!(commands.len(), 2);
+    assert_eq!(commands[0], "ls -la");
+    assert_eq!(commands[1], "cat file.txt");
+}
+
+#[test]
+fn test_extract_shell_commands_no_section() {
+    let response = "Some response without the expected section.";
+    let commands = extract_shell_commands(response);
+    assert!(commands.is_empty());
+}
+
+#[test]
+fn test_extract_shell_commands_multiple_code_blocks() {
+    let response = r#"
+{{CODE EXPLORATION COMMANDS}}
+
+```bash
+ls -la
+```
+
+Some explanation text.
+
+```
+cat README.md
+head -50 src/main.rs
+```
+"#;
+
+    let commands = extract_shell_commands(response);
+    assert_eq!(commands.len(), 3);
+    assert_eq!(commands[0], "ls -la");
+    assert_eq!(commands[1], "cat README.md");
+    assert_eq!(commands[2], "head -50 src/main.rs");
+}