fix(scripts): improve git branch creation error handling

[grafvonb]

Summary

Improves error handling and efficiency in create-new-feature scripts (bash and PowerShell) when git checkout -b fails.

Changes

Capture git error output

Previously, git checkout -b stderr was discarded (2>/dev/null), making it impossible to diagnose why branch creation failed. Now the error output is captured and surfaced to the user.

Skip redundant checkout when already on target branch

When --allow-existing-branch is used and the user is already on the target branch, the scripts previously attempted an unnecessary git checkout. Now they detect this case and skip the checkout entirely.

Surface actual git error messages

When branch creation fails for reasons other than "branch already exists" (e.g., detached HEAD, dirty worktree), the actual git error message is now displayed instead of a generic fallback.

Files changed

• create-new-feature.sh
• create-new-feature.ps1

Testing

All 43 existing tests pass (5 PowerShell runtime tests skipped — expected without pwsh). Key test classes verified:

• TestAllowExistingBranch — 8 tests covering switch-to-branch, already-on-branch, spec dir creation, no-overwrite, JSON output, and no-git scenarios
• TestDryRun, TestTimestampBranch, TestSequentialBranch — no regressions

AI Disclosure

• I did not use AI assistance for this contribution

• I did use AI assistance (describe below)

• Code generated, reviewed and tested by Copilot with Claude Opus 4.6

[Copilot]

Pull request overview

Improves the robustness of the create-new-feature helper scripts by surfacing git checkout -b failure details and avoiding redundant checkouts when an existing target branch is allowed.

Changes:

• Capture and display git checkout -b error output instead of discarding it.
• When --allow-existing-branch / -AllowExistingBranch is set, skip an unnecessary git checkout if already on the target branch.
• Improve fallback error messages by including actual Git output when branch creation fails for non-“already exists” reasons.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
scripts/bash/create-new-feature.sh	Captures branch-create stderr, detects already-on-branch, and prints more informative failure output.
scripts/powershell/create-new-feature.ps1	Captures branch-create error output, avoids redundant checkout when already on the target branch, and surfaces Git failure details.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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