feat: add NovitaAI as LLM provider

Add NovitaAI as a first-class provider with dedicated model selection
flow, live pricing, and authoritative context length resolution.

- Register provider in PROVIDER_REGISTRY, HERMES_OVERLAYS, and all
  alias/label maps (ID: novita, aliases: novita-ai, novitaai)
- Add dedicated _model_flow_novita() with 3-tier model list fallback:
  Novita API → models.dev → static curated list
- Fetch live pricing from /v1/models with correct unit conversion
  (input_token_price_per_m is 0.0001 USD per Mtok)
- Add Novita-specific context length resolution (step 4b) in
  get_model_context_length(), prioritized over models.dev/OpenRouter
- Register api.novita.ai in _URL_TO_PROVIDER to prevent early return
  from the custom-endpoint code path
- Add models.dev mapping (novita → novita-ai)
- Add default auxiliary model (deepseek/deepseek-v3-0324)
- Add NOVITA_API_KEY to test isolation (conftest.py)
- Update docs: providers page, env vars reference, CLI reference,
  .env.example, README, and landing page

.env.example

@@ -14,6 +14,14 @@
 # LLM_MODEL is no longer read from .env — this line is kept for reference only.
 # LLM_MODEL=anthropic/claude-opus-4.6
 
+# =============================================================================
+# LLM PROVIDER (NovitaAI)
+# =============================================================================
+# NovitaAI — multi-model aggregator with pay-per-use pricing
+# Get your key at: https://novita.ai/settings/key-management
+# NOVITA_API_KEY=
+# NOVITA_BASE_URL=https://api.novita.ai/openai/v1  # Override default base URL
+
 # =============================================================================
 # LLM PROVIDER (Google AI Studio / Gemini)
 # =============================================================================