fix(git): surface checkout stderr for existing branches

[PascalThuet]

Summary

This PR preserves Git's original stderr when create-new-feature fails while switching to an existing branch in the --allow-existing-branch flow.

Changes

• capture git checkout stderr in the Bash and PowerShell scripts
• apply the same fix to the extensions/git script variants
• add a regression test that reproduces a real checkout failure caused by local changes
• add parity guards for the git extension script copies

Testing

• pytest tests/test_timestamp_branches.py

Context

Follow-up to #2089.

Fixes #2105.

[Copilot]

Pull request overview

This PR improves error diagnosability in the --allow-existing-branch flow by preserving and surfacing git checkout stderr when switching to an already-existing branch fails (e.g., due to local changes), and adds regression/parity tests to prevent reintroducing stderr suppression.

Changes:

• Capture and print git checkout stderr on existing-branch switch failures in Bash and PowerShell scripts.
• Apply the same behavior to the extensions/git script copies for parity.
• Add a regression test reproducing a real-world checkout failure caused by conflicting local changes, plus static parity guards for extension script variants.

Show a summary per file

File	Description
tests/test_timestamp_branches.py	Adds regression test for checkout failure stderr surfacing + static parity guards for main/extension script variants.
scripts/powershell/create-new-feature.ps1	Captures git checkout stderr when switching to an existing branch and includes it in the error output.
scripts/bash/create-new-feature.sh	Captures git checkout stderr when switching to an existing branch and prints it on failure.
extensions/git/scripts/powershell/create-new-feature.ps1	Mirrors PowerShell stderr-surfacing behavior for extension parity.
extensions/git/scripts/bash/create-new-feature.sh	Mirrors Bash stderr-surfacing behavior for extension parity.

Copilot's findings

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• Files reviewed: 5/5 changed files
• Comments generated: 0