refactor(g3-core): extract 4 modules from monolithic lib.rs

Reduce lib.rs from 7481 to 6557 lines (-12.4%) by extracting:

- paths.rs: Session/workspace path utilities (get_todo_path, get_logs_dir, etc.)
- streaming_parser.rs: StreamingToolParser for LLM response parsing
- utils.rs: Diff parsing and shell escaping utilities
- webdriver_session.rs: Unified Safari/Chrome WebDriver abstraction

All public APIs preserved via re-exports for backward compatibility.
Added 13 new unit tests across extracted modules.
All 225 tests pass.

crates/g3-core/src/paths.rs

@@ -0,0 +1,124 @@
+//! Path utilities for G3 session and workspace management.
+//!
+//! This module centralizes all path-related logic for:
+//! - TODO file location
+//! - Logs directory
+//! - Session directories and files
+//! - Thinned content storage
+
+use std::path::PathBuf;
+
+/// Environment variable name for workspace path.
+/// Used to direct all logs to the workspace directory.
+pub const G3_WORKSPACE_PATH_ENV: &str = "G3_WORKSPACE_PATH";
+
+/// Environment variable name for custom TODO file path.
+const G3_TODO_PATH_ENV: &str = "G3_TODO_PATH";
+
+/// Get the path to the todo.g3.md file.
+///
+/// Checks for G3_TODO_PATH environment variable first (used by planning mode),
+/// then falls back to todo.g3.md in the current directory.
+pub fn get_todo_path() -> PathBuf {
+    if let Ok(custom_path) = std::env::var(G3_TODO_PATH_ENV) {
+        PathBuf::from(custom_path)
+    } else {
+        std::env::current_dir().unwrap_or_default().join("todo.g3.md")
+    }
+}
+
+/// Get the path to the logs directory.
+///
+/// Checks for G3_WORKSPACE_PATH environment variable first (used by planning mode),
+/// then falls back to "logs" in the current directory.
+pub fn get_logs_dir() -> PathBuf {
+    if let Ok(workspace_path) = std::env::var(G3_WORKSPACE_PATH_ENV) {
+        PathBuf::from(workspace_path).join("logs")
+    } else {
+        std::env::current_dir().unwrap_or_default().join("logs")
+    }
+}
+
+/// Public accessor for the logs directory path (for use by submodules).
+/// Alias for `get_logs_dir()` for backward compatibility.
+pub fn logs_dir() -> PathBuf {
+    get_logs_dir()
+}
+
+/// Get the base .g3 directory path.
+/// This is the root for all g3 session data in the current workspace.
+pub fn get_g3_dir() -> PathBuf {
+    if let Ok(workspace_path) = std::env::var(G3_WORKSPACE_PATH_ENV) {
+        PathBuf::from(workspace_path).join(".g3")
+    } else {
+        std::env::current_dir().unwrap_or_default().join(".g3")
+    }
+}
+
+/// Get the session directory for a specific session ID.
+/// Returns .g3/sessions/<session_id>/
+pub fn get_session_logs_dir(session_id: &str) -> PathBuf {
+    get_g3_dir().join("sessions").join(session_id)
+}
+
+/// Ensure the session directory exists for a specific session ID.
+/// Creates .g3/sessions/<session_id>/ and subdirectories.
+pub fn ensure_session_dir(session_id: &str) -> std::io::Result<PathBuf> {
+    let session_dir = get_session_logs_dir(session_id);
+    std::fs::create_dir_all(&session_dir)?;
+
+    // Create subdirectories
+    std::fs::create_dir_all(session_dir.join("thinned"))?;
+
+    Ok(session_dir)
+}
+
+/// Get the thinned content directory for a session.
+/// Returns .g3/sessions/<session_id>/thinned/
+pub fn get_thinned_dir(session_id: &str) -> PathBuf {
+    get_session_logs_dir(session_id).join("thinned")
+}
+
+/// Get the path to the session.json file for a session.
+/// Returns .g3/sessions/<session_id>/session.json
+pub fn get_session_file(session_id: &str) -> PathBuf {
+    get_session_logs_dir(session_id).join("session.json")
+}
+
+/// Get the path to the context summary file for a session.
+/// Returns .g3/sessions/<session_id>/context_summary.txt
+pub fn get_context_summary_file(session_id: &str) -> PathBuf {
+    get_session_logs_dir(session_id).join("context_summary.txt")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_todo_path_default() {
+        // When G3_TODO_PATH is not set, should return current_dir/todo.g3.md
+        std::env::remove_var(G3_TODO_PATH_ENV);
+        let path = get_todo_path();
+        assert!(path.ends_with("todo.g3.md"));
+    }
+
+    #[test]
+    fn test_session_paths_are_consistent() {
+        let session_id = "test-session-123";
+        let session_dir = get_session_logs_dir(session_id);
+        let thinned_dir = get_thinned_dir(session_id);
+        let session_file = get_session_file(session_id);
+        let summary_file = get_context_summary_file(session_id);
+
+        // All paths should be under the session directory
+        assert!(thinned_dir.starts_with(&session_dir));
+        assert!(session_file.starts_with(&session_dir));
+        assert!(summary_file.starts_with(&session_dir));
+
+        // Check expected filenames
+        assert!(thinned_dir.ends_with("thinned"));
+        assert!(session_file.ends_with("session.json"));
+        assert!(summary_file.ends_with("context_summary.txt"));
+    }
+}