Context Builder UI: select files and preview exact prompt context

[jamesrochabrun]

Parent: #398

Depends on: Context Builder core selection and assembly model.

Goal

Add a launcher-facing context curation UI where the user can select files and preview the exact context block that will be handed to the agent.

Implementation Notes

Target area: MultiSessionLaunchView / MultiSessionLaunchViewModel, plus a small focused ContextBuilderView and ContextBuilderViewModel.

Use existing FileIndexService.rootNodes, children, and readFile behavior for project tree data. The initial UI should stay small:

• button in the launcher: Curate Context
• tree/list of project files with multi-select
• selected-files list
• assembled-context preview
• clear/remove selection controls

Do not build line slicing, profiles, or code maps in this issue. Leave hooks/placeholders where useful, but keep the first UI shippable.

Acceptance Criteria

• User can open context curation from the launcher.
• User can select and deselect files from the current repository.
• User can preview the exact assembled context block.
• Launching a session includes the assembled context before the prompt.
• Existing attached-file behavior is not broken.

Tests

Add view-model tests for selection updates and assembly handoff. Keep UI components small and composable.