opencode prices and Go limits, settings out of .env

- Fetch prices for opencode (Zen and Go) incl. limits, image input, reasoning,
  long-context tiers and peak-hour/date factors; costs apply them per request
- Record the cost opencode reports per response; track Go allowance in 5h/7d/month
  windows against a provider plan limit (budget table, status overlay)
- Settings → Config file: move values of ${NAME} references into the config
- Examples, presets and forms keep keys in the config instead of .env

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/provider/openai.rs

@@ -231,6 +231,7 @@ impl Provider for OpenAi {
                 usage.input_tokens = u["prompt_tokens"].as_u64().unwrap_or(0).saturating_sub(cached);
                 usage.cache_read_tokens = cached;
                 usage.output_tokens = u["completion_tokens"].as_u64().unwrap_or(0);
+                usage.read_reported_cost(u);
             }
             let Some(choice) = data["choices"].get(0) else { continue };
             let d = &choice["delta"];