Feature request: option to auto-create feature branch for new proposals

Summary

It would be useful if OpenSpec could optionally create a new feature branch in the git repository whenever a new proposal is started through the /openspec-proposal slash command or equivalent. This behaviour is inspired by SpecKit’s branch workflow: each new change begins on its own branch with a numeric prefix and an AI-generated slug.

Motivation

Today, when starting a new proposal with OpenSpec, all changes are made in the current branch unless the user manually creates a feature branch. Creating a dedicated branch per feature makes it easier to manage work in progress, track history, and review pull requests. SpecKit implements this pattern by generating branch names like 001-add-profile-filters, 002-update-auth, etc. Bringing a similar capability to OpenSpec would improve developer ergonomics.

Proposal

• Add a configuration option (e.g. in openspec/project.md or a CLI flag) that enables automatic branch creation when starting a new proposal.
• When enabled, OpenSpec should:
  1. Determine the next sequence number (e.g. based on existing feature branches in the repo).
  2. Generate a human-readable branch name based on the proposal title (using AI to slugify).
  3. Create and check out the new branch before scaffolding the openspec/changes/<change>/ directory.
• The user should still have the ability to disable this feature if they prefer to manage branches manually.

Benefits

• Each feature or change lives in its own branch, making it easier to see the history of development.
• Aligns with existing workflows in tools like SpecKit, making migration smoother.
• Reduces manual steps and potential mistakes when starting a new feature.

Please consider adding this optional branch creation mechanism to OpenSpec to streamline the workflow.