Add ai review configuration for fallback and timeouts

[kergoth]

Current path now:

• per-source ai_timeout supported via git-sources.yml
• pass-through works update-git-lock -> update/update.ps1 -> show-git-changes

Future option clean:

1. Keep per-source overrides

• ai_agent, ai_model, ai_timeout in each source entry
• best when one source always same reviewer config

2. Add central matrix (better for fallback chains)

• new config file: agent+model -> timeout
• lookup order:
  • explicit source ai_timeout
  • matrix exact (agent,model)
  • matrix agent default
  • global default
• enables chain like claude -> codex -> pi(local) with proper timeouts per hop

3. Add reviewer chain config

[kergoth]

Best to make the new config file general for ai review config beyond the per-source level, so capture both preferred agent fallback chain and agent/model timeout in one config file.