add package.json config source with numeric priority system

[clavery]

Summary

This PR introduces a new PackageJsonSource configuration source that reads project-level defaults from the b2c key in package.json, along with a numeric priority system for fine-grained control over config source ordering.

Features

Package.json Configuration Source

• Reads configuration from the b2c key in package.json
• Has the lowest priority (1000) of all config sources - won't override dw.json or CLI flags
• Only allows non-sensitive, project-level fields:
  • shortCode - SCAPI short code
  • clientId - OAuth client ID (for implicit login discovery)
  • mrtProject - MRT project slug
  • mrtOrigin - MRT API origin URL
  • accountManagerHost - Account Manager hostname
• Sensitive fields (passwords, secrets, hostname) are intentionally excluded for security

Example package.json:

{
  "name": "my-storefront",
  "b2c": {
    "shortCode": "abc123",
    "clientId": "your-client-id",
    "mrtProject": "my-project"
  }
}

Numeric Priority System

Config sources now support numeric priorities for fine-grained ordering:

Priority	Description	Example
< 0	Override built-in sources	Plugin 'before' → -1
0	Built-in sources	DwJsonSource, MobifySource
1-999	After built-in	Plugin 'after' → 10
1000	Lowest priority	PackageJsonSource

• Existing 'before'/'after' string values continue to work (mapped to -1 and 10)
• Plugin hooks can now return numeric priority values
• Custom ConfigSource implementations can set priority property directly

Files Changed

Implementation

• packages/b2c-tooling-sdk/src/config/types.ts - Added priority?: number to ConfigSource
• packages/b2c-tooling-sdk/src/config/sources/package-json-source.ts - New source
• packages/b2c-tooling-sdk/src/config/sources/dw-json-source.ts - Added priority = 0
• packages/b2c-tooling-sdk/src/config/sources/mobify-source.ts - Added priority = 0
• packages/b2c-tooling-sdk/src/config/resolver.ts - Sort by priority, add PackageJsonSource
• packages/b2c-tooling-sdk/src/cli/hooks.ts - Updated type for numeric priority
• packages/b2c-tooling-sdk/src/cli/base-command.ts - Map string priorities to numeric

Tests

• packages/b2c-tooling-sdk/test/config/sources.test.ts - PackageJsonSource tests
• packages/b2c-tooling-sdk/test/config/resolver.test.ts - Priority sorting tests

Documentation

• docs/guide/configuration.md - Added package.json section, updated priority list
• docs/guide/extending.md - Added numeric priority documentation

[clavery]

bypassing as we want this in for plugin priority