fix(ci): prevent pipefail SIGPIPE in workflow validation

[jaysin586]

Prevents workflow validation from failing with exit code 141 when a repository has more than five branches. The diagnostic branch listing now uses sed, which consumes the complete input stream and remains compatible with set -o pipefail.

Replaces git branch -a | head -5 with git branch -a | sed -n '1,5p' in .github/workflows/validate-workflows.yml. Unlike head, sed reads the entire input stream, so the upstream git branch never receives SIGPIPE and genuine validation failures are still detected under set -euo pipefail.

🤖 Generated with Claude Code

[coderabbitai]

Important

Review skipped

Auto reviews are limited based on label configuration.

🏷️ Required labels (at least one) (1)

• coderabbit

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b329b4f8-81d5-4227-b274-38651caac57a

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

_{Comment @coderabbitai help to get the list of available commands.}