Add explicit model specifications to commands and agents

- Add model: sonnet to issue-worker agent (balanced for implementation)
- Add model: sonnet to pr-fixer agent (balanced for feedback iteration)
- Add model: haiku to /dashboard command (read-only display)
- Add model: haiku to /roadmap command (read-only categorization)
- Document rationale for each model selection in frontmatter comments

Closes #72

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

agents/pr-fixer/agent.md

@@ -1,6 +1,10 @@
 ---
 name: pr-fixer
 description: Autonomous agent that addresses PR review feedback in an isolated git worktree
+# Model: sonnet provides balanced speed and capability for addressing feedback.
+# Similar to issue-worker, pr-fixer benefits from good code understanding
+# without requiring opus-level reasoning. Quick iteration on review feedback.
+model: sonnet
 tools: Bash, Read, Write, Edit, Glob, Grep, TodoWrite, Task
 skills: gitea, code-review
 ---