feat: support `default-status-checks` at repository level

[rnetser]

Problem

default-status-checks is global-only. Different repos have different external CI checks (GitHub Actions, third-party CI) that should be required for can-be-merged, but there's no way to configure this per-repo.

Proposed Solution

Add default-status-checks to the per-repository schema, same as how security-checks, ai-features, etc. are available at both levels. Per-repo overrides global.

Config example

# Global
default-status-checks:
  - "WIP"
  - "can-be-merged"

repositories:
  my-repo:
    name: my-org/my-repo
    # Per-repo — overrides or extends global
    default-status-checks:
      - "WIP"
      - "can-be-merged"
      - "ci/my-external-check"

Done

• Add default-status-checks to per-repo schema in webhook_server/config/schema.yaml
• Update get_required_status_checks() to merge/override global with per-repo
• Update examples/config.yaml with per-repo example
• Add tests
• Update docs