Add agent guidance and opt-in GitHub release migration

[samuel-williams-shopify]

Make modernization aware of gems using bake-gem-github, and provide agent guidance for updating a gem and reviewing the generated changes.

Separate commits cover:

• Ignore author names ending in [bot] in copyright and gem author lists, preserving human attribution through bot renames.
• Add the opt-in modernize:releases:github task to migrate dependencies and release hooks. Regular modernization omits the redundant publishing hook and keeps the correct readme release command when bake-gem-github is declared. Existing bake.rb files continue to use the Ollama merge, with instructions to remove the obsolete publishing call and preserve custom behavior.
• Publish a getting-started guide as installable agent context, including selective review, verification, and the separate GitHub setup/plan/apply steps.
• Upgrade this repository to bake-gem-github ~> 0.5.0 and regenerate release validation and review rules. Release PRs must contain one commit; ordinary PRs can contain multiple commits and use rebase merging when enabled in repository settings.

The migration is outside the default task and does not change remote settings.

Validation: 72 tests / 169 assertions pass; Covered reports 502/502 lines exercised. RuboCop and documentation coverage (40/40 definitions) pass. The gem builds with the context and template included, and agent:context:install discovers the new guide. Regenerated release files are stable on a second setup update.