feat(worktrees): make worktree creation prompt-first

[fsioni]

Why

Worktree creation currently starts with configuration and setup, then asks for the task. This PR makes it prompt-first: describe the work, optionally attach context, and let Jean create/setup the worktree before starting the conversation automatically.

What changed

• one compact composer with project, model, mode, attachments, skills, base branch, and Create more
• existing Issues, PRs, Security, Branches, Linear, and Sentry flows preserved as context pickers
• every worktree creation entry point now opens the same prompt-first flow
• queued prompts survive long setup scripts and reloads, then send exactly once
• empty-prompt worktree creation remains supported
• responsive mobile flow with sidebar dismissal and model selection

Preview

Validation

• 85 prompt-first unit tests
• 24 end-to-end workflow tests across desktop and mobile

I have also been using this flow for three days with normal worktrees, PRs, issues, attachments, skills, Create more, and long setup scripts.

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[fsioni]

@andrasbacsai I've been using it for a week now. Really changed my workflow for the best