Check remote branches to prevent duplicate branch numbers

[sigent-amazon]

Summary

This PR adds remote branch checking to prevent duplicate branch numbers when creating new features. The current implementation only checks local specs/ directories, which can lead to duplicate branch numbers when branches exist on remotes (e.g., in open pull requests) but not locally.

Motivation

When working in a team environment, developers may have feature branches on remotes that don't exist locally. The current branch numbering logic only scans the local specs/ directory, which can result in:

• Duplicate branch numbers (e.g., two different feature/001-* branches)
• Merge conflicts when trying to push branches
• Confusion about which feature number corresponds to which work

This PR solves this by checking all three sources: remote branches, local branches, and specs directories.

Changes

• Added --number parameter to create-new-feature.sh and create-new-feature.ps1 for manual branch number specification
• Added check_existing_branches() function (bash) and Get-NextBranchNumber function (PowerShell) to fetch and scan remote branches
• Checks three sources for existing feature numbers: remote branches (via git ls-remote), local branches, and specs directories
• Uses exact short-name pattern matching ([0-9]+-<short-name>$) to prevent false positives
• Updated /speckit.specify command documentation to include remote branch checking workflow
• Prevents duplicate branch numbers when branches exist on remotes but not locally
• Maintains backward compatibility with existing workflows
• Falls back to local directory scanning when Git is not available

Testing

• Tested with existing remote branches - correctly identifies highest number
• Tested with no remote branches - starts at 001 as expected
• Tested with --number parameter override - manual number specification works
• Tested fallback to local directory scanning (no-git mode) - works without Git
• Tested exact pattern matching - doesn't match similar branch names
• Tested with branches in specs directories only - finds correct number
• Backward compatible with existing workflows - no breaking changes

Checklist

• I have read the contributing guidelines
• I have tested my changes locally
• I have updated documentation where necessary
• My changes follow the project's coding conventions
• I have added tests for new functionality (bash and PowerShell scripts)
• All existing tests pass
• My commit messages follow best practices

AI Assistance Disclosure

This PR was developed with assistance from Amazon Q Developer CLI. The implementation was:

• Designed and architected by human review of the existing codebase
• Code generated with AI assistance for bash and PowerShell implementations
• Thoroughly tested and validated by human review
• Documentation and PR description written with AI assistance
• All changes reviewed and understood by the contributor

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