governance: add local link gate to pre-commit

[ChBrain]

Summary

• Add local link integrity check in pre-commit for staged regions markdown files
• Keep local validation aligned with CI link checks
• Clarify pre-commit validation chain comment to match actual orchestrator layers

Why

• Trust, but verify: checks that can run locally should fail locally before push
• Prevent CI-only surprise failures for broken markdown links

Validation

• python -m pytest -q tests/test_links.py -k nigeria
• python -m pytest -q tests/test_hook_scope_e2e.py -k world_branch_allows_anything_in_regions