chore(backup): track repo-root .backupignore — it is the single source of truth now (FPLAN-0269 follow-up)

.backupignore is now AIPass's managed backup filter (true gitignore semantics via pathspec), so it belongs in version control like .gitignore — a fresh clone gets the curated rules, not just the auto-seed default. Includes the .ruff_cache/ + .coverage additions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.backupignore

@@ -0,0 +1,28 @@
+# Backup System ignore patterns (gitignore-style)
+# Lines starting with # are comments. Blank lines are ignored.
+# Edit this file to customize. Source defaults: handlers/ignore/patterns.py
+
+.backup_system/
+.git/
+.svn/
+.hg/
+__pycache__/
+.pytest_cache/
+*.pyc
+*.pyo
+*.egg-info/
+.venv/
+venv/
+.tox/
+node_modules/
+.vscode/
+.idea/
+*.swp
+*.swo
+.DS_Store
+Thumbs.db
+build/
+dist/
+*.log
+.ruff_cache/
+.coverage