Value opencode Go plan requests; per-duration cache prices; usage under answers

- opencode Go reports plan requests as cost 0 with the usage in allowance;
  ModelConfig::cost values them at the model's list prices (charged amounts
  and free requests stay as reported). Rows booked at 0 are valued once on
  start (idempotent).
- opencode's price list now has per-duration cache prices for some models
  ({"30m": ..., "12h": ...}): the shortest duration is used, also in tiers.
  Saving the provider's models failed with "untagged enum ModelEntry".
- Each answer shows tokens sent/received and the cost of the turn's requests
  (usage.cost filled when messages are saved and loaded).
- The chat title keeps a minimum width in a crowded top bar.

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

core/src/config.rs

@@ -159,7 +159,13 @@ impl ModelConfig {
     /// Cost of one request now. Cache prices default to the input price.
     /// The provider's own figure wins when it reports one (opencode).
     pub fn cost(&self, u: &crate::types::Usage) -> f64 {
-        u.cost.unwrap_or_else(|| self.cost_at(u, chrono::Utc::now()))
+        match u.cost {
+            // A plan request (opencode Go): nothing charged, the usage counts toward the plan –
+            // its value at the model's list prices.
+            Some(c) if c == 0.0 && u.allowance.is_some_and(|a| a > 0.0) => self.cost_at(u, chrono::Utc::now()),
+            Some(c) => c,
+            None => self.cost_at(u, chrono::Utc::now()),
+        }
     }
 
     /// Cost of one request at a given time: context tiers and time factors applied.
@@ -187,6 +193,21 @@ mod price_tests {
         Usage { input_tokens: input, output_tokens: output, cache_read_tokens: read, ..Default::default() }
     }
 
+    #[test]
+    fn plan_requests_are_valued_at_list_prices() {
+        let m = ModelConfig { input_price: Some(1.0), output_price: Some(2.0), ..Default::default() };
+        let mut u = usage(1_000_000, 1_000_000, 0);
+        u.cost = Some(0.0);
+        u.allowance = Some(0.4);
+        assert_eq!(m.cost(&u), 3.0);
+        // Charged by the provider: that amount; free without a plan: free.
+        u.cost = Some(0.7);
+        assert_eq!(m.cost(&u), 0.7);
+        u.cost = Some(0.0);
+        u.allowance = None;
+        assert_eq!(m.cost(&u), 0.0);
+    }
+
     #[test]
     fn tiers_and_multipliers() {
         let m = ModelConfig {