feat: add Google Gemini provider support

[0Baris]

What

Adds Google Gemini as a fourth AI provider alongside OpenAI, Ollama, and
Claude Code. Default model is gemini-2.5-flash-lite.

Changes

New provider

• src/providers/gemini.ts — GeminiProvider using @google/generative-ai
  with temperature: 0.3 and maxOutputTokens: 200

Wiring

• src/providers/types.ts — added 'gemini' to ProviderOptions.provider
  union type
• src/providers/index.ts — factory routes to GeminiProvider, reads
  GEMINI_API_KEY env var
• src/index.ts — added 'gemini' to RewriteOptions.provider union type,
  added gemini-2.5-flash-lite to default model resolution chain, fixed
  execCommand to use stdio: ['pipe', 'pipe', 'pipe']

CLI

• src/cli.ts — updated --provider description, --api-key description,
  --model description; added Gemini API key validation error, informational
  message, help text, and usage examples

Other

• .env.example — added GEMINI_API_KEY entry
• package.json — added @google/generative-ai ^0.24.1 dependency

Usage

export GEMINI_API_KEY="your-api-key"
git-rewrite-commits --provider gemini

# or with a specific model
git-rewrite-commits --provider gemini --model gemini-2.5-flash-lite