* feat(scripts): add --allow-existing-branch flag to create-new-feature
Add an --allow-existing-branch / -AllowExistingBranch flag to both
bash and PowerShell create-new-feature scripts. When the target branch
already exists, the script switches to it instead of failing. The spec
directory and template are still created if missing, but existing
spec.md files are not overwritten (prevents data loss on re-runs).
The flag is opt-in, so existing behavior is completely unchanged
without it. This enables worktree-based workflows and CI/CD pipelines
that create branches externally before running speckit.specify.
Relates to #1931. Also addresses #1680, #841, #1921.
Assisted-By: 🤖 Claude Code
* fix: address PR review feedback for allow-existing-branch
- Make checkout failure fatal instead of suppressing with || true (bash)
- Check $LASTEXITCODE after git checkout in PowerShell
- Use Test-Path -PathType Leaf for spec file existence check (PS)
- Add PowerShell static assertion test for -AllowExistingBranch flag
Assisted-By: 🤖 Claude Code
