Add create-pr.sh helper script for streamlined PR creation #36

Problem

Creating PRs requires remembering the correct gh pr create flags (repo, base, head, title, body). This is repetitive and error-prone.

Solution

Add a create-pr.sh shell script at the repository root that automates PR creation with sensible defaults:

• Derives PR title from the branch name (kebab-case → sentence)
• Auto-generates PR body from commit log since master
• Includes a test-plan checklist
• Accepts optional positional arguments [title] [body] for overrides
• Guards against accidentally running on master

Usage

# Auto-derive title and body
./create-pr.sh

# Override title only
./create-pr.sh "My PR title"

# Override both
./create-pr.sh "My PR title" "Custom body"

Acceptance Criteria

• create-pr.sh exists at repo root
• Script is executable (chmod +x)
• Script fails with a clear error when run on master
• Auto-derived title converts branch kebab-case to sentence form
• Auto-derived body includes commits summary and test checklist