[#54]

Add init command for guided setup and config management improvements.

New Commands

1. newrelic-cli init - Interactive Setup Wizard

$ newrelic-cli init
New Relic CLI Setup

API Key (NRAK-...): ****
Account ID: 12345
Region (US/EU) [US]: US

Testing connection...
✓ API key valid
✓ Account 12345 accessible

Configuration saved.

Try it out:
  newrelic-cli apps list
  newrelic-cli nrql "SELECT count(*) FROM Transaction"

Features:

• Interactive prompts for API key, account ID, region
• Connectivity verification (unless --no-verify)
• Existing config detection with overwrite warning
• Non-interactive mode with --api-key, --account-id, --region flags

2. newrelic-cli config test - Connectivity Verification

$ newrelic-cli config test
Testing connection to New Relic...

Region: US

✓ API key valid
  User: user@example.com
✓ Account 12345 accessible
  Name: My Account
✓ NerdGraph API responding

✓ Connection test passed!

3. newrelic-cli config clear - Remove All Credentials

$ newrelic-cli config clear
Clear all stored credentials (API key, account ID, region)? [y/N]: y
✓ Cleared API key from Keychain
✓ Cleared account ID from Keychain
✓ Cleared region setting

Note: Environment variables (NEWRELIC_*) will still be used if set.

Test Plan

• Build passes: make build
• Tests pass: make test
• init --help shows correct usage
• config test --help shows correct usage
• config clear --help shows correct usage
• Interactive init works with prompts
• Non-interactive init works with flags: init --api-key X --account-id Y --region US --no-verify
• config test verifies credentials
• config clear removes all credentials

Closes #54