Introduce a Provider interface selectable via CPM_CADDY_MODE:
- CaddyfileProvider renders per-host snippets (template whitespace fixed)
- APIProvider manages Caddy's JSON config via the admin API
Add configurable reload strategy (systemd/exec/api/none) and a
configurable listen address for the bootstrapped server in API mode.
Includes unit tests for the JSON builder, provider factory and the
API provider (httptest).