Harvest PR #3010: lock slim default prompt with calm-overlay regression test

The release branch already ships the slim-prompt intent in a stronger
form than #3010 — compose_default_static_layers appends no personality
overlay at all (voice/tone guidance is folded into the constitution
preamble), where the PR only emptied the Calm arm. Carry over the PR's
regression test, which derives its assertion from calm.md itself so a
future re-inclusion of the overlay text fails loudly.

Harvested from PR #3010 by @894876246

Co-authored-by: Hanmiao Li <894876246@qq.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/tui/src/prompts.rs

@@ -3182,4 +3182,18 @@ mod tests {
             "Concise Output Discipline should be appended"
         );
     }
+
+    /// #2953 — the Calm overlay (calm.md) stays out of the default
+    /// model-prompt path to keep the static prefix slim. Voice and tone
+    /// guidance travels via the constitution preamble instead.
+    #[test]
+    fn default_prompt_does_not_include_calm_personality_overlay() {
+        let prompt = compose_prompt(Personality::Calm);
+        let calm_text = include_str!("prompts/personalities/calm.md");
+        let first_calm_line = calm_text.lines().find(|l| !l.is_empty()).unwrap_or("");
+        assert!(
+            !prompt.contains(first_calm_line),
+            "default agent prompt must not include calm.md overlay"
+        );
+    }
 }