estimate prompt tokens on restore so status bar isn't 0K

After restore_from_log + compact, set last_prompt_tokens from
the budget's used() count instead of waiting for the first API call.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/runner.rs

@@ -1003,6 +1003,9 @@ impl Agent {
             all.len(), mem_count, conv_count);
         self.context.entries = all;
         self.compact();
+        // Estimate prompt tokens from budget so status bar isn't 0 on startup
+        let b = self.budget();
+        self.last_prompt_tokens = b.used() as u32;
         true
     }