Add more granular CI review matrix capability

Currently you can run multiple review types and multiple agents, but you can't run multiple review types with different agents for each review. Suggest evolving to support also a config.toml syntax like

```
[ci]
enabled = true
poll_interval = "1m"
repos = [...]
synthesis_agent = "codex"
min_severity = "medium"

[ci.reviews]
codex = ["security", "review"]
gemini = ["review"]
```