feat: integrate opencode CLI provider with automatic model fallback a…

[fcmiranda]

feat: integrate opencode CLI provider with automatic model fallback and update configuration workflows

Summary

This PR adds opencode as a first-class provider for commit message and PR title generation, makes it the default provider, and updates config and docs to support CLI-based workflows without API keys.

Main Features

• Added new opencode provider integration for both commit and pr commands.
• Implemented automatic model fallback for opencode execution:
  • Tries configured primary model first.
  • Falls back through configured free-model candidates if a model fails.
• Switched default provider to opencode in configuration defaults.
• Added provider-level fallback_models support in config schema.
• Updated interactive config flow to:
  • Include opencode as a selectable provider.
  • Skip API key and endpoint prompts for CLI-backed providers (anthropic, gemini, opencode).
  • Support opencode model selection and num_suggestions.
• Expanded provider model catalog with opencode free model entries.
• Added tests for opencode model candidate normalization and deduplication.
• Updated README with:
  • New provider list and default behavior.
  • opencode setup guidance and troubleshooting note.

Files Changed

• README.md
• cmd/commit.go
• cmd/config.go
• cmd/pr.go
• internal/config/config.go
• internal/provider/models/models.go
• internal/provider/opencode.go
• internal/provider/opencode_test.go

Why

• Provides a zero-API-key default path using the opencode CLI and free models.
• Improves resilience by retrying across fallback models when the first choice fails.
• Aligns configuration UX with CLI-backed provider behavior.

Notes

• opencode CLI must be installed and authenticated for default flow.
• Existing non-CLI providers remain available (copilot, openai, anthropic, gemini).