fix(scripts): suppress git fetch stdout leak in multi-remote environments

[seiya-koji]

Description

In multi-remote environments, git fetch --all outputs messages like Fetching origin to stdout. Since
check_existing_branches() only redirected stderr (2>/dev/null), the stdout output was captured by
$(check_existing_branches ...) command substitution, contaminating the branch number return value and
causing arithmetic errors like $((10#Fetching...)).

Fix: redirect both stdout and stderr to /dev/null (>/dev/null 2>&1).

Reproduction

1. Clone a repository and add a second remote (e.g. a fork)
2. Run create-new-feature.sh
3. git fetch --all outputs Fetching origin\nFetching fork to stdout
4. The output is captured into the variable that should contain only a branch number
5. Script fails with a bash arithmetic error

Actual error output

Before fix (2>/dev/null — stderr only):

$ result="$(git fetch --all --prune 2>/dev/null; echo "10")"
$ echo "$result"
Fetching origin
Fetching fork
10

$ BRANCH_NUMBER="$result"
$ echo "$((10#$BRANCH_NUMBER))"
(eval):1: bad math expression: operator expected at `Fetching o...'

After fix (>/dev/null 2>&1 — stdout + stderr):

$ result="$(git fetch --all --prune >/dev/null 2>&1; echo "10")"
$ echo "$result"
10

$ BRANCH_NUMBER="$result"
$ echo "$((10#$BRANCH_NUMBER))"
10

Testing

• Confirmed the bug in a multi-remote environment (repository with both origin and a fork remote)
• Verified git fetch --all --prune >/dev/null 2>&1 suppresses stdout in multi-remote setup
• Ran create-new-feature.sh after the fix and confirmed branch numbering works correctly

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

The bug was discovered automatically when running Spec Kit's specify command via GitHub Copilot
(Claude Opus 4.6) in a downstream project with multiple git remotes. The fix was also generated by the
same agent.

[Copilot]

Pull request overview

This PR fixes a bash scripting bug in create-new-feature.sh where git fetch --all --prune could emit remote “Fetching …” lines to stdout in multi-remote setups, contaminating command-substitution output used for numeric branch calculations.

Changes:

• Redirect git fetch --all --prune stdout + stderr to /dev/null to keep check_existing_branches() output purely numeric.

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[Copilot]

Pull request overview

This PR fixes a bash scripting bug where git fetch --all stdout could leak into command-substitution output in multi-remote repositories, corrupting the numeric branch-prefix value used for feature branch numbering.

Changes:

• Redirect git fetch --all --prune stdout + stderr to /dev/null to prevent contaminating check_existing_branches() output.
• Preserve existing behavior of tolerating fetch failures (|| true) while ensuring the function returns a clean numeric value.

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