Add replaces support to sync script for downstream file cleanup

[yvonnedevlinrh]

Context

PR #390 converted issue templates from .md to .yml. The old .md files will remain as orphans in downstream repos since the sync script can only push new files — it has no mechanism to remove superseded ones.

This affects any future file rename or format migration synced via sync-config.yml.

Suggested implementation

Add a replaces key to sync-config.yml entries that instructs the sync script to delete the old file in downstream repos when pushing the replacement:

- source: .github/ISSUE_TEMPLATE/bug_report.yml
  destination: .github/ISSUE_TEMPLATE/bug_report.yml
  replaces: .github/ISSUE_TEMPLATE/bug_report.md

The sync script would include the deletion in the same PR that pushes the new file.

References

• Implement new YAML format for issues forms #54 (parent: convert issue templates to YAML forms)
• feat: convert issue templates from Markdown to YAML forms #390 (PR that introduced the .md → .yml migration)

[marcusburghardt]

#396 is should also have a related impact here.