Support for OpenRouter

[LeeSinLiang]

Add OpenRouter provider

Summary
Implement an OpenRouter provider backend so users can route to many hosted models with one key.

Scope

• Provider id: openrouter.
• Config/env:
  • OPENROUTER_API_KEY
  • Optional OPENROUTER_BASE_URL (default https://openrouter.ai/api/v1)
  • model id (e.g., openrouter/anthropic/claude-3.5-sonnet, openrouter/qwen/qwen2.5-coder:latest)
• Support in:
  • commitlm init provider selection.
  • commitlm generate --provider openrouter --model <id>.
• Respect existing fallback behavior to local HF when errors/timeouts occur.

Tasks

• Define provider adapter (auth header, timeouts/retries, error mapping).
• Unit tests with mocked HTTP for success/timeout/429.
• Update README configuration matrix and examples.
• Optional: list a few tested model ids in docs.

Acceptance criteria

• commitlm validate confirms connectivity for OpenRouter.
• Generation works for commit message and docs paths.
• Clear errors on auth/rate-limit failures; fallback honored when configured.

Test plan
Set OPENROUTER_API_KEY; run commitlm generate --provider openrouter --model <id> in a temp repo; verify outputs.