fix: exclude sysprompt etc from context left % (#2446)

- Prevents the % left indicator from immediately decrementing to ~97%.
- Tested by prompting "hi" and noting it only decremented to 99%. And by
adding a bunch of debug logs and observing numbers.

codex-rs/protocol/src/protocol.rs

@@ -508,6 +508,33 @@ impl TokenUsage {
         self.total_tokens
             .saturating_sub(self.reasoning_output_tokens.unwrap_or(0))
     }
+
+    /// Estimate the remaining user-controllable percentage of the model's context window.
+    ///
+    /// `context_window` is the total size of the model's context window.
+    /// `baseline_used_tokens` should capture tokens that are always present in
+    /// the context (e.g., system prompt and fixed tool instructions) so that
+    /// the percentage reflects the portion the user can influence.
+    ///
+    /// This normalizes both the numerator and denominator by subtracting the
+    /// baseline, so immediately after the first prompt the UI shows 100% left
+    /// and trends toward 0% as the user fills the effective window.
+    pub fn percent_of_context_window_remaining(
+        &self,
+        context_window: u64,
+        baseline_used_tokens: u64,
+    ) -> u8 {
+        if context_window <= baseline_used_tokens {
+            return 0;
+        }
+
+        let effective_window = context_window - baseline_used_tokens;
+        let used = self
+            .tokens_in_context_window()
+            .saturating_sub(baseline_used_tokens);
+        let remaining = effective_window.saturating_sub(used);
+        ((remaining as f32 / effective_window as f32) * 100.0).clamp(0.0, 100.0) as u8
+    }
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]