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>

cmd_system.go

@@ -67,6 +67,13 @@ Export/Import:
     --skip-existing          - Skip existing keys (default)
     --overwrite              - Overwrite existing keys
 
+Shell Scripting:
+  set <name> <value>         - Set shell variable
+  unset <name>               - Remove shell variable
+  vars                       - List all variables
+    Variables: $VAR or ${VAR}
+    Environment: $ENV:VARNAME
+
 Batch Operations:
   batch <file>               - Execute commands from file
   source <file>              - Alias for batch