feat: add shell scripting with variables

Implemented roadmap #9: Shell scripts execution with variables.

Features:
- set <name> <value> - Set shell variable
- unset <name> - Remove variable
- vars - List all variables
- Variable substitution in all commands

Variable syntax:
- $VAR or ${VAR} - Shell variables
- $ENV:VARNAME - Environment variables

Variables expand before command execution, enabling:
- Dynamic key paths: put $BASE/$ID '{"data":"value"}'
- Reusable values: set TOKEN xyz && auth $TOKEN
- Script parameterization in batch files

Example batch script:
  set USER alice
  set KEY_PREFIX users
  put $KEY_PREFIX/$USER '{"name":"$USER"}'
  get $KEY_PREFIX/$USER

Variables are session-scoped and work in both interactive
and batch modes. Environment variables can be injected using
$ENV:HOME syntax.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

main.go

@@ -10,6 +10,9 @@ import (
 
 // executeCommand executes a single command line
 func (c *KVSClient) executeCommand(line string) bool {
+	// Expand variables in the line
+	line = c.expandVariables(line)
+
 	parts := parseCommand(line)
 	if len(parts) == 0 {
 		return true
@@ -66,6 +69,12 @@ func (c *KVSClient) executeCommand(line string) bool {
 		c.handleImport(args)
 	case "export-list":
 		c.handleExportList(args)
+	case "set":
+		c.handleSet(args)
+	case "unset":
+		c.handleUnset(args)
+	case "vars":
+		c.handleVars(args)
 	case "batch", "source":
 		c.handleBatch(args, c.executeCommand)
 	default: