Add documentation and tooling for creating sub-issues for REST API endpoints

[Copilot]

Issue #236 requests sub-issues for each endpoint in the GitHub REST API Sub-Issues documentation. This PR provides templates and automation to create those 5 sub-issues.

Files Added

• docs/MANUAL_SUB_ISSUE_CREATION.md - Complete issue templates for all 5 endpoints (Get parent, Remove, List, Add, Reprioritize) with requirements following docs/script-generation-rules.md
• docs/create-sub-issues-plan.md - Endpoint overview and metadata
• tmp/README.md - Usage instructions for automation script
• tmp/create-sub-issues-for-endpoints.sh - Bash script to create all issues via GitHub API and link them to parent Add support for sub issue REST API #236

Implementation

Each sub-issue template includes:

• Endpoint documentation URL from https://docs.github.com/en/enterprise-cloud@latest/rest/issues/sub-issues
• HTTP method and path
• Target script filename
• Requirements checklist from script generation rules
• script-generation label

Three creation methods provided:

1. Manual via GitHub web interface using provided templates
2. Automated via bash script (requires .gh-api-examples.conf with GITHUB_TOKEN)
3. GitHub CLI with example commands

Script automatically links created issues as sub-issues to #236 using POST /repos/{owner}/{repo}/issues/{issue_number}/sub_issues.

Original prompt

For all the endpoints in https://docs.github.com/en/enterprise-cloud@latest/rest/issues/sub-issues?apiVersion=2022-11-28 create a sub issue in this issue with the rules in docs/script-generation-rules.md

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