feat(output): add custom output format, test coverage, and release 2.1.0

[dshevtsov]

Purpose

Adds a custom output format backed by Mustache templates, closes out test coverage gaps across the gem, hardens the CI lint workflow, and bumps the version to 2.1.0.

What changed and why

• Custom output format: Added CustomFormatter, which renders PR rows through a project-supplied Mustache template (templates.custom config key, path validated to stay inside the project directory) and writes to custom_output. Markdown output uses the bundled Mustache template while retaining the existing four-column, headerless layout and whitespace behavior. YAML retains object-based to_yaml serialization. Row#to_h supplies the full row fields for YAML and custom templates; Markdown builds its own four-field template data.
• Test coverage: Added RSpec specs for Config, Client, EnterpriseClient, Pulls, RowCollector, Row, Runner, CLI, and the output formatters (98 examples), plus SimpleCov and WebMock (network calls disabled in specs).
• Cucumber scenarios: Added network-free scenarios covering invalid --config paths, all output formats, missing custom_output, and the enterprise SSRF guard. Added VCR-backed scaffolding (tagged @vcr, excluded from the default run) with a recorded cassette for replay-only testing against a real GitHub repository.
• CI lint hardening: Fixed all super-linter failures surfaced across two CI runs:
  • Scoped workflow permissions per job, bumped actions/checkout to v6 and super-linter to v8, and pinned both to commit SHAs (zizmor).
  • Removed unrecognized RuboCop disable directives and trailing whitespace; deduplicated near-identical spec setup to satisfy JSCPD's 0% duplicate-code threshold.
  • Bumped faraday and json in Gemfile.lock to patched versions (Trivy CVEs).
  • Raised the gemspec's required_ruby_version to >= 3.3 so RuboCop's auto-detected TargetRubyVersion matches the Ruby 3.1+ hash-punning syntax used in specs.
  • Disabled VALIDATE_JSON_PRETTIER (conflicted with Biome's formatting of .markdownlint.json).
  • Symlinked .rubocop.yml to the existing .github/linters/.ruby-lint.yml and added rubocop-github as a development dependency, so CI and local/IDE RuboCop share the same config.
• Docs: Documented the custom output format and the new test tooling (SimpleCov, WebMock, VCR, cucumber -p vcr) in the README, and added a CHANGELOG entry.
• Version bump: 2.0.0 -> 2.1.0.
• Compatibility changes: The minimum supported Ruby version increases from 3.0 to 3.3, dropping support for Ruby 3.0-3.2. Row#date and generated row dates now use ISO 8601 (YYYY-MM-DD) rather than human-readable dates; Row#date_string provides the human-readable form. Consumers that parse the previous date format must adapt.
• Housekeeping: Ignored local AI-assistant directories (.claude/, .scout/) in .gitignore.

Testing

• bundle exec rspec — 98 examples, 0 failures
• bundle exec rubocop lib spec — 0 offenses
• bundle exec cucumber — 10 scenarios, 10 passed (default, network-free)
• bundle exec cucumber -p vcr — 1 scenario, replays from cassette with no live network access
• bundle exec rake test — full suite green
• CI (super-linter) — green