Add instance management commands and easy setup

[clavery]

TODO: setup cli or something for initial setup (perhaps trigger when no config can be sourced and setup is run 🤔 )

Summary

• Rename setup config → setup inspect
• Add setup instance list to view all configured instances
• Add setup instance create with interactive prompts for creating instance configs
• Add setup instance remove to delete instance configurations
• Add setup instance set-active to set the default instance

Architecture

Two Capabilities Model

The design separates two distinct capabilities that config sources can provide:

Capability	Purpose	Example Sources
Instance Source	Stores full instance config (hostname, name, code-version)	dw.json, global registry, cloud config
Credential Source	Stores secrets only, linked by instance name	Keychain plugin, Vault plugin

At resolution time, values merge: dw.json provides hostname + keychain provides password.

ConfigSource Interface Extension

Sources can now implement optional instance management methods:

interface ConfigSource {
  // Existing
  name: string;
  priority?: number;
  load(options: ResolveConfigOptions): ConfigLoadResult | undefined;

  // Instance Management (for dw.json-style sources)
  listInstances?(options?: ResolveConfigOptions): InstanceInfo[];
  createInstance?(options: CreateInstanceOptions & ResolveConfigOptions): void;
  removeInstance?(name: string, options?: ResolveConfigOptions): void;
  setActiveInstance?(name: string, options?: ResolveConfigOptions): void;

  // Credential Storage (for keychain-style sources)
  credentialFields?: (keyof NormalizedConfig)[];
  storeCredential?(instanceName: string, field: keyof NormalizedConfig, value: string, options?: ResolveConfigOptions): void;
  removeCredential?(instanceName: string, field: keyof NormalizedConfig, options?: ResolveConfigOptions): void;
}

Plugin Extensibility

Plugins can implement these methods to:

1. Provide instance storage - Store instances in alternative locations (cloud config, global registry)
2. Provide credential storage - Securely store secrets in keychains, vaults, etc.
3. Aggregate across sources - InstanceManager class aggregates listInstances() from all sources

Example plugin implementation:

// A keychain plugin can implement:
class KeychainSource implements ConfigSource {
  name = 'keychain';
  credentialFields = ['password', 'clientSecret'];
  
  storeCredential(instanceName, field, value) {
    // Store in OS keychain keyed by instance name
  }
  
  load(options) {
    // Load credentials from keychain for the requested instance
  }
}

This PR implements DwJsonSource as the first instance source. The InstanceManager service aggregates instances from all sources that implement listInstances().

Implementation

• Extended ConfigSource interface with optional instance management and credential storage methods
• Implemented CRUD operations in DwJsonSource for managing the configs[] array in dw.json
• Added InstanceManager service class to aggregate across multiple sources
• Added mapNormalizedConfigToDwJson() function for reverse mapping
• Exported DwJsonSource class for direct use by plugins

Test plan

• Unit tests for dw-json CRUD functions
• Unit tests for DwJsonSource instance methods
• Existing tests updated for rename (setup config → setup inspect)
• Manual testing of CLI commands:

  b2c setup inspect
  b2c setup instance list
  b2c setup instance create staging --hostname staging.example.com
  b2c setup instance set-active staging
  b2c setup instance remove staging --force