Update agents to use explicit model selection

Both agents now use `sonnet` instead of `inherit`:

- code-reviewer: Sonnet provides good code understanding for
  structured review tasks without needing opus-level reasoning
- product-manager: Planning and issue-writing follow patterns
  from skills; sonnet handles these well

Each agent includes a comment explaining the model choice.

Closes #18

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agents/code-reviewer/AGENT.md

@@ -1,7 +1,9 @@
 ---
 name: code-reviewer
 description: Automated code review of pull requests. Reviews PRs for quality, bugs, security, style, and test coverage. Spawn after PR creation or for on-demand review.
-model: inherit
+# Model: sonnet provides good code understanding for review tasks.
+# The structured output format doesn't require opus-level reasoning.
+model: sonnet
 skills: forgejo, code-review
 ---