Create worktree dialog ignores the Default Agent set in Settings #185

Problem

The Create worktree dialog does not preselect the agent chosen in Settings → Agents → Default Agent. A worktree created without changing the agent picker starts with a different agent from the one the user set as their default.

Expected

When the dialog opens, its agent picker shows the Default Agent from Settings, if that agent is available. After the user changes the setting, the next dialog that opens uses the new value. If the default agent is unavailable, the dialog falls back as it does now and says why.

Steps to reproduce

1. Open Settings → Agents and set Default Agent to an agent other than the built-in default, for example a Claude chat.
2. Open the Create worktree dialog for a project.
3. Look at the agent picker, or create the worktree without touching it.

Actual: the picker shows another agent, and the worktree's chat starts with that agent.

Leads (not yet verified)

• apps/web/src/components/CreateWorktreeDialog.tsx:366 reads the default from serverConfigs.get(environmentId), the project's environment settings. Settings edits go through useUpdatePrimarySettings (SettingsPanels.tsx:427). A project on another environment, or a cached server config that has not picked up the change, would resolve a different default.
• At submit (CreateWorktreeDialog.tsx:445-454) the fallback chain ends at project.defaultModelSelection and then codex. If the actions list is empty when the dialog opens, the picker and the created thread both miss the setting.
• CreateWorktreeDialog.test.tsx:678 and :712 cover a default set before the dialog mounts. No test covers changing the setting while the app is running and then opening the dialog.

Acceptance

• A test that changes Default Agent at runtime and then opens the dialog. It asserts the preselected agent and the provider of the created thread.
• The dialog and Settings read the default from the same source.