chore: remove dead branch-protection.json from sync-config

## Problem

`.github/branch-protection.json` is listed in `sync-config.json` (line 42) and synced to every fork, but no workflow or script reads it. Branch protection is applied through `.github/rulesets/*.json` by `settings-apply` and initialization. The file still carries `required_linear_history: true`, which ADR-009 records as removed, so a reader who finds it gets the wrong answer.

## Required change

1. Delete `.github/branch-protection.json`.
2. Remove its entry from `sync-config.json` and add it to `cleanup_rules` so existing forks drop the file on their next template sync.

Start by inspecting .github/branch-protection.json and the entry for it in sync-config.json. Check the cleanup_rules format and the references to .github/rulesets/*.json, then confirm that the obsolete file is deleted, removed from sync entries, and listed for cleanup on the next template sync.

Written by the indexing model from the issue text.