Harden Merge and Automation Policy

[Copilot]

Closes#4

The repository lacked documented and enforceable merge/automation policy: no explicit merge strategy, no named required status checks, and underspecified branch protection guidance.

Changes

docs/REPOSITORY_MANAGEMENT.md

• New Branch Protection section enumerating the minimum rules for main
• New Required Status Checks table mapping the exact GitHub check name Repository hygiene / validate to .github/workflows/repository-hygiene.yml
• New Merge Strategy section specifying squash-only merges on main

CONTRIBUTING.md

• Pull requests section now states that all PRs are squash-merged using the PR title and description

TEMPLATE_CHECKLIST.md

• Replaced the generic branch-protection item with actionable items that name Repository hygiene / validate and call out squash-merge configuration explicitly

CHANGELOG.md

• Records the policy documentation additions under [Unreleased]