feat(plugins): Bundler plugin — bundle contribute entry point

[cdhagmann]

Per ADR-0014, ship a Bundler plugin that delegates to the same dispatch table as the standalone CLI.

Scope:

• Add plugins.rb at the gem root per Bundler plugin convention
• Register a bundle contribute command via Bundler::Plugin::API
• Plugin entry point MUST NOT load Rooibos or ratatui_ruby (TUI is gated to the standalone binary)
• Bare bundle contribute runs scan (resolved — matches bundle fund behavior)
• bundle contribute <verb> mirrors gem-contribute <verb>
• Smoke test: bundle plugin install gem-contribute against the local gem dispatches a verb

See: ADR-0014, ROADMAP v1.x