feat: support local file path in extends (both commit-helper and commithelper-go)

[kyungmi]

Summary

@naverpay/commit-helper (Node.js) and @naverpay/commithelper-go (Go) extends only supports HTTP/HTTPS URLs. Neither implements local file path support.

Resolved in #83 (Go) and #84 (Node.js).

@naverpay/commit-helper (Node.js) supports an extends field that fetches and merges a remote config. @naverpay/commithelper-go did not implement this feature.

Resolved in #83.

Motivation

Teams with multiple repositories want to share a common .commithelperrc.json baseline (branch-prefix rules, protect lists, Jira passthrough) and override only project-specific rules locally.

Implemented Behavior

extends accepts either an HTTP/HTTPS URL or a local file path:

Remote URL:

{
    "extends": "https://raw.githubusercontent.com/my-org/.github/main/.commithelperrc.json",
    "rules": {
        "my-feature": "my-org/my-repo"
    }
}

Local file path (recommended for private registries — install shared config as a dev dependency):

{
    "extends": "./node_modules/@my-org/commithelperrc/.commithelperrc.json",
    "rules": {
        "my-feature": "my-org/my-repo"
    }
}

Merge Strategy

Field	Strategy
rules	base is default; local key wins on conflict
protect	union (base ∪ local), deduped
passthrough	union (base ∪ local), deduped
template	local wins; falls back to base if unset
extends in base	ignored — no recursive loading

Load failure (file not found, HTTP error, invalid JSON) is a fatal error (exit 1).

Reference

Node.js implementation: packages/commit-helper/bin/cli.ts — readExternalConfig() function