gateway: capture real provider-reported cost (openrouter usage accounting)

Cost displays were estimates from a pricing table; on OpenRouter the
status bar never reflected what was actually charged. Now cost is
provider-REPORTED only, end to end:

- OpenRouter requests carry usage:{include:true} (profile + legacy
  transport paths); the response usage.cost field (credits, 1:1 USD)
  is captured per call into agent.session_actual_cost_usd and
  persisted to the sessions DB actual_cost_usd column (NULL-safe:
  unreported calls never touch the stored value).
- Nous keeps its x-nous-credits-* header capture; the header delta
  now surfaces as the session's real cost via real_session_cost_usd.
- Providers that report nothing accumulate NOTHING: cost fields stay
  absent/None (the TUI hides its cost segment), never a fabricated
  $0.00 and never an estimate. _get_usage, gateway /usage and the
  CLI usage page all switched off estimate_usage_cost for display.
- Per-model session accumulator (session_model_usage) records real
  per-call counts and provider-reported cost per model.

run_agent.py

@@ -632,6 +632,9 @@ class AIAgent:
         self.session_reasoning_tokens = 0
         self.session_api_calls = 0
         self.session_estimated_cost_usd = 0.0
+        # Provider-REPORTED cost only — None means "nothing reported".
+        self.session_actual_cost_usd = None
+        self.session_model_usage = {}
         self.session_cost_status = "unknown"
         self.session_cost_source = "none"