Add multi-provider support (Claude, OpenCode, Cursor, Kiro)

[tyzerrr]

Summary

Abstract the CLI provider behind a common interface to support multiple AI backends.

Context

The 99 plugin supports OpenCode, Claude Code, Cursor Agent, and Kiro via a provider abstraction. cc99 currently only supports Claude Code CLI.

Acceptance Criteria

• Provider interface/base class
• Claude Code provider (existing, refactored)
• At least one additional provider
• Configurable provider selection in setup()