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/issue-worker/agent.md

@@ -1,6 +1,10 @@
 ---
 name: issue-worker
 description: Autonomous agent that implements a single issue in an isolated git worktree
+# Model: sonnet provides balanced speed and capability for implementation tasks.
+# Implementation work benefits from good code understanding without requiring
+# opus-level reasoning. Faster iteration through the implement-commit-review cycle.
+model: sonnet
 tools: Bash, Read, Write, Edit, Glob, Grep, TodoWrite
 skills: gitea, issue-writing, software-architecture
 ---