Add release immutability support to repository plugin

[Copilot]

The repository plugin had no support for GitHub's release immutability feature, which prevents assets and tags from being modified after a release is published. This adds releases.immutable as an enforceable setting via safe-settings.

Changes

• lib/plugins/repository.js

  • Added releases to ignorableFields (not a repos.update param; handled separately)
  • Extracts this.releases in constructor, removes it from this.settings before API calls
  • New updateReleaseImmutability() method: calls PUT /repos/{owner}/{repo}/releases/immutability to enable, DELETE to disable; supports nop/dry-run mode
  • sync() calls updateReleaseImmutability() in both changed and unchanged repo branches

• test/unit/lib/plugins/repository.test.js

  • Added github.request mock; fixed mock repo data to include owner.login/name
  • Removed a pre-existing it.only that was silently skipping all other tests
  • New tests for enable, disable, and absent releases config

• Docs: Added releases.immutable to settings.yml sample and 1. repository-settings.md

Usage

repository:
  releases:
    immutable: true   # PUT /repos/{owner}/{repo}/releases/immutability
    # immutable: false  # DELETE /repos/{owner}/{repo}/releases/immutability