Add per-task model selection for executor

[bborn]

Summary

• Tasks can now have a model override (e.g., opus, sonnet, haiku) that gets passed via --model flag to the Claude CLI
• Adds a Model selector to the task creation/edit form (only shown for executors that support multiple models — currently Claude)
• Adds executor and model parameters to the MCP taskyou_create_task tool so agents can specify model when creating tasks

How it works

• New model column in the tasks database (empty string = use executor's default model)
• When a task has a model set, --model <value> is passed to the Claude CLI in all execution paths: initial run, resume, dangerous mode toggle, safe mode toggle, and interactive BuildCommand
• The form UI shows a model selector (default / opus / sonnet / haiku) when the Claude executor is selected. Other executors show no model picker since they don't support --model
• Model selection cycles with left/right arrows and supports type-to-select by first letter

Test plan

• All existing tests pass (go test ./...)
• Create a task with model set to "opus" — verify --model opus appears in the tmux command
• Create a task with no model selected — verify no --model flag is added
• Switch executor from Claude to Codex — verify model picker disappears
• Edit an existing task — verify model persists correctly
• Create a task via MCP tool with model parameter — verify it's stored and used

🤖 Generated with Claude Code