feat: add variable management CLI command

[Hinne1]

What & Why

Adds the missing n8nctl variable command (alias: var) for managing n8n environment variables from the CLI. Previously we had to use raw curl calls against the n8n API to manage variables.

Key Changes

• New command: n8nctl variable with list, get, create, update, delete subcommands
• All subcommands resolve by key name (not ID) for ergonomics
• Supports --json output on list and get
• Registered in root command with var alias

Usage

n8nctl var list                          # Table output
n8nctl var list --json                   # JSON output
n8nctl var get mts_stargate_url          # Get value by key
n8nctl var create my_key "my_value"      # Create
n8nctl var update my_key "new_value"     # Update
n8nctl var delete my_key                 # Delete

Testing

• go build ./... — clean
• Manually tested full CRUD lifecycle on stage

[gemini-code-assist]

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