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>

utils.go

@@ -130,6 +130,9 @@ var completer = readline.NewPrefixCompleter(
 	readline.PcItem("export"),
 	readline.PcItem("import"),
 	readline.PcItem("export-list"),
+	readline.PcItem("set"),
+	readline.PcItem("unset"),
+	readline.PcItem("vars"),
 	readline.PcItem("batch"),
 	readline.PcItem("source"),
 	readline.PcItem("help"),