fix(ci): skip Rust builds on infra-only changes, remove stale files
Add `rules:` with `changes:` filters to check, test, and .kaniko-base
jobs so the Rust build pipeline only triggers when source code changes
(crates/, bin/, services/, Cargo.*). Prevents H100 spin-up on
infra-only pushes.
Remove 31 stale/orphan files (-11,885 lines):
- monitoring/ directory (duplicated by config/grafana + config/prometheus)
- 10 broken/placeholder migration files (.broken, .skip)
- 10 unreferenced config files (haproxy, nginx-lb, mutants, etc.)
Add 3 historical design docs from previous restructure work.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
ac688cac24