feat(homecore-server): seed 13 default services across 6 domains on boot

Operators (and the new web UI) saw "No services registered" on every
vanilla boot because nothing in the boot sequence called
`ServiceRegistry::register()`. The Assist pipeline registers intent
handlers — a different surface — but `/api/services` stayed empty
until a plugin or integration loaded.

Adds `seed_default_services()` after `HomeCore::new()`. Each handler
is a `FnHandler` that echoes the call back as a JSON acknowledgement
so the service registry is exercise-able from day one. Integrations
override these by re-registering the same `ServiceName` with a real
handler later.

Seeded set:

  homeassistant: restart, stop, reload_core_config
  light:         turn_on, turn_off, toggle
  switch:        turn_on, turn_off, toggle
  scene:         apply
  automation:    trigger
  homecore:      ping, snapshot_state   (HOMECORE-native)

Boot log now reports:

  Service registry seeded with 13 default service(s)

GET /api/services now returns 6 domains with 13 services total.
The HOMECORE web UI's Services page shows them under proper
domain headings.

Co-Authored-By: claude-flow <ruv@ruv.net>