CaddySpec had only `enable` left (path_prefix/host removed; extra_snippets was
unused), so `proxy: { caddy: {} }` was three levels of nesting for a checkbox —
and it invited drift (supabase silently reverted to proxy.caddy.host). Replace it
with a plain `ServiceSpec.proxy: bool`.
- Model: remove ProxySpec/CaddySpec; `proxy: bool = False`. Readers simplified to
`bool(svc.proxy)` (generator, castle-api summaries, CLI info).
- CLI create / dashboard editor + create form: write `proxy: true` / a checkbox.
- Configs migrated to `proxy: true` (and supabase's stale proxy.caddy.host block
removed).
- Docs (registry, dns-and-tls, design, stack guides) + tests/fixtures updated;
also fixed lingering path-model staleness in the stack guides.
- Introduced `_write_castle_config` function to scatter nested config into separate YAML files for programs, services, and jobs.
- Updated `castle_root` fixture to utilize the new config writing method.
- Added tests for configuration aggregation and scattering in `test_health.py`.
- Removed legacy unit handling from `manifest.py` and refactored related code in `config.py`.
- Updated documentation to reflect new configuration directory structure.
- Removed obsolete unit tests related to unit expansion.
