refactor: Consolidate REPL command descriptions to single source

- Removed duplicate getCommandDescription function from completer.go
- Added package-level getCommandDescription in repl.go as single source of truth
- Updated defaultGetCommandDescription to delegate to getCommandDescription
- Created minimal completer.go that uses getCommandDescription for test compatibility

Command descriptions are now defined only once, eliminating duplication
between the original completer.go and defaultGetCommandDescription in repl.go.

The refactoring maintains:
  - Backward compatibility (tests still work)
  - Consistent descriptions across the codebase
  - Single source of truth for command descriptions

2 files changed, 10 insertions, 17 deletions

diff --git a/internal/repl/completer.go b/internal/repl/completer.go
index e9387fd..3f55212 100644
--- a/internal/repl/completer.go
+++ b/internal/repl/completer.go
@@ -50,20 +50,3 @@ func completer(d prompt.Document) []prompt.Suggest {
 	}
 	return suggestions
 }
-
-// getCommandDescription returns the description for a built-in command.
-// It's used by the completer function to provide helpful descriptions during tab-completion.
-//
-// cmd: the built-in command name (e.g., "help", "clear", "quit")
-// Returns the description string for the command, or empty string if not found
-func getCommandDescription(cmd string) string {
-	descriptions := map[string]string{
-		"help":  "Show help information",
-		"clear": "Clear the screen",
-		"quit":  "Exit the REPL",
-		"exit":  "Exit the REPL",
-		"rpn":   "Evaluate an RPN (postfix notation) expression",
-		"calc":  "Same as rpn - evaluate an RPN expression",
-	}
-	return descriptions[cmd]
-}
diff --git a/internal/repl/repl.go b/internal/repl/repl.go
index 905b390..950156b 100644
--- a/internal/repl/repl.go
+++ b/internal/repl/repl.go
@@ -182,6 +182,16 @@ func defaultCompleter(r *REPL, d prompt.Document) []prompt.Suggest {
 // cmd: the built-in command name (e.g., "help", "clear", "quit")
 // Returns the description string for the command, or empty string if not found
 func (r *REPL) defaultGetCommandDescription(cmd string) string {
+	return getCommandDescription(cmd)
+}
+
+// getCommandDescription returns the description for a built-in command.
+// This is a package-level function that provides a single source of truth
+// for command descriptions, used by defaultGetCommandDescription.
+//
+// cmd: the built-in command name (e.g., "help", "clear", "quit")
+// Returns the description string for the command, or empty string if not found
+func getCommandDescription(cmd string) string {
 	descriptions := map[string]string{
 		"help":  "Show help information",
 		"clear": "Clear the screen",