Add model selection to workflow definitions

[alari76]

Summary

• Adds optional model field to workflow MD frontmatter (e.g. model: claude-sonnet-4-6)
• Exposes model selection (Opus/Sonnet/Haiku) as button group in both Add Workflow and Edit Workflow modals
• Model flows through config → schedule input → session creation → Claude CLI --model flag
• Per-config model (UI) takes priority over per-definition model (frontmatter), with system default as final fallback

Test plan

• Open Add Workflow modal → verify Model button group appears on Step 3 (Schedule)
• Select Sonnet, create workflow → verify config persists model: claude-sonnet-4-6
• Open Edit Workflow modal for existing workflow → verify model field reflects saved value
• Change model in edit modal → save → verify updated in config
• Trigger workflow manually → verify Claude spawns with correct --model flag
• Leave model as "Default (Opus)" → verify no --model flag is passed

🤖 Generated with Claude Code