providers: comfyui missing from _VALID_PROVIDERS despite img.toml provider="comfyui"

[thinmintdev]

Pre-existing latent gap surfaced during #620 (PR #647) review.

src/hal0/config/schema.py _VALID_PROVIDERS does not include comfyui, but the seeded img.toml uses provider = \"comfyui\". This is present on main (not introduced by #647) — a SlotConfig validation gap for image slots.

Fix: add comfyui to _VALID_PROVIDERS (and confirm no other seeded provider identifier is missing). Add a test asserting every seeded slot TOML provider is in _VALID_PROVIDERS.

Found-by: triage sweep 2026-06-07.