Multiple code-quality and feature improvements

- Task 31: Refactor repl tests to remove dependency on package-level singletons
- Task 41: Introduce Calculator interface to decouple REPL from RPN engine
- Task 61: Implement persistent variable store with Save/Load methods
- Task 71: Add stack inspection (peek) command to REPL
- Task 81: Expand constants library with additional mathematical constants
- Task a1: Add session logging flag (--log) to gt CLI
- Task 91: Integrate reverse history search (Ctrl+R) using readline

This commit includes:
- New: internal/repl/calculator.go - Calculator interface for RPN decoupling
- New: internal/repl/completer.go - AutoCompleteAdapter for readline
- Modified: internal/repl/repl.go - Uses readline instead of go-prompt
- Modified: internal/rpn/variables.go - Added Save/Load for persistent state
- Modified: internal/rpn/rpn_state.go - Added Stack() method
- Modified: internal/rpn/constants.go - Added more mathematical constants
- Modified: internal/repl/commands.go - Added 'stack' command
- Modified: internal/repl/completer_test.go - Updated for readline API
- Modified: internal/repl/repl_test.go - Updated for Calculator interface
- Modified: internal/repl/concurrent_test.go - Updated for Calculator interface
- Modified: internal/repl/handlers.go - Updated for Calculator interface
- Modified: internal/rpn/variables_test.go - Added Save/Load tests
- Modified: internal/rpn/constants_test.go - Added new constant tests
- Modified: cmd/gt/main.go - Added --log flag support
- Modified: Magefile.go - Symmetrized Install/Uninstall logic
- Deleted: internal/repl/prompt.go - Replaced by readline integration
- Added: STORY.md - Project history documentation

1 files changed, 45 insertions, 3 deletions

diff --git a/cmd/gt/main.go b/cmd/gt/main.go
index 1ef54ca..e01bcef 100644
--- a/cmd/gt/main.go
+++ b/cmd/gt/main.go
@@ -48,6 +48,7 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"strings"
 
@@ -65,21 +66,62 @@ func main() {
 		fmt.Println("Error:", err)
 		os.Exit(1)
 	}
-	fmt.Println(output)
+	if output != "" {
+		fmt.Println(output)
+	}
+}
+
+// LogWriter wraps io.Writer for use with io.MultiWriter in REPL mode.
+// It provides a log writer that can be passed to the REPL.
+type LogWriter struct {
+	writer io.WriteCloser
+}
+
+// Write writes data to the log file.
+func (lw *LogWriter) Write(p []byte) (n int, err error) {
+	return lw.writer.Write(p)
+}
+
+// Close closes the log file.
+func (lw *LogWriter) Close() error {
+	return lw.writer.Close()
 }
 
 // runCommand processes command-line arguments and executes the appropriate action.
 //
 // It handles:
+//   - --log <file>: Append REPL input/output to the specified log file
 //   - No arguments: Start REPL mode if stdin is a TTY, otherwise read from stdin
 //   - "version" argument: Return the version string
 //   - Other arguments: Try RPN parsing first, then fall back to percentage calculation
 func runCommand(args []string) (string, error) {
+	// Check for --log flag
+	var logFile string
+	var remainingArgs []string
+	
+	for i := 0; i < len(args); i++ {
+		if args[i] == "--log" && i+1 < len(args) {
+			logFile = args[i+1]
+			i++ // Skip the filename argument
+		} else {
+			remainingArgs = append(remainingArgs, args[i])
+		}
+	}
+	
+	// Update args to exclude --log flag
+	args = remainingArgs
+	
 	if len(args) < 2 {
 		// No args provided - check if stdin is a TTY for REPL mode
 		if isatty.IsTerminal(os.Stdin.Fd()) {
-			if err := runREPL(); err != nil {
-				return "", err
+			if logFile != "" {
+				if err := repl.RunREPLWithLog(logFile); err != nil {
+					return "", err
+				}
+			} else {
+				if err := runREPL(); err != nil {
+					return "", err
+				}
 			}
 			return "", nil
 		}