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>

commands/dashboard.md

@@ -1,5 +1,8 @@
 ---
 description: Show dashboard of open issues, PRs awaiting review, and CI status.
+# Model: haiku is sufficient for fetching and displaying data.
+# This command only reads from Gitea and formats output - no complex reasoning needed.
+model: haiku
 ---
 
 # Repository Dashboard

commands/roadmap.md

@@ -1,5 +1,9 @@
 ---
 description: View current issues as a roadmap. Shows open issues organized by status and dependencies.
+# Model: haiku is sufficient for fetching and organizing issue data.
+# This command reads issues and dependencies, then displays them in categories.
+# Basic categorization logic doesn't require advanced reasoning.
+model: haiku
 argument-hint:
 ---