feat(homecore-server): seed 10 default entities on boot (--no-seed-entities to opt out)

Companion to the seed_default_services() commit. Dashboard + States
pages now have content on every fresh --db :memory: boot, not just
after `bash scripts/homecore-seed.sh`.

Adds:
  - new CLI flag `--no-seed-entities` (default: enabled)
  - `seed_default_entities(hc)` mirroring the bash script's 10-entity
    set (4 RuView sensing-derived + 6 conventional HA fixtures)
  - Boot log:
        Service registry seeded with 13 default service(s)
        State machine seeded with 10 default entities

Two seeds stay in sync — integrations overwrite the same entity_ids
via /api/states/<id> POST. Run with --no-seed-entities when wiring
real plugins that populate the state machine themselves.

Empirical (after rebuild + fresh restart):
  GET /api/states   → 10 entities
  GET /api/services → 6 domains, 13 services

homecore-server --db :memory: is now enough for the web UI to be
fully populated on first paint.

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