feat(ai): add OpenCode Go provider

[aarsh21]

Summary

• add opencode-go as a supported AI provider
• route OpenCode Go to the OpenAI-compatible https://opencode.ai/zen/go/v1/ endpoint
• document Go model IDs and add CLI parsing coverage
• initialize git test repos with main so merge-base tests are deterministic

Tests

• cargo check
• cargo test (127 passed)
• cargo run -- --provider opencode-go --model kimi-k2.6 draft --help
• cargo run -- --provider opencode-go diff --help