feat(ai-skills): add shared compose_system_prompt helper

crates/op-ai-skills/src/compose.rs

@@ -0,0 +1,65 @@
+//! The single skill-text expander shared by the desktop transport and the
+//! backend proxy. Maps skill NAMES to their markdown content, concatenates
+//! in order, and trims to a character budget so the same names always
+//! yield byte-identical prompts on every host.
+
+use crate::loader::get_skill_by_name;
+
+/// Compose a system prompt from skill names. Unknown names are skipped
+/// (the corpus is the source of truth). `budget_chars` caps the result
+/// length (0 = unlimited); when exceeded, whole skills are dropped from
+/// the END so earlier (higher-priority) skills survive intact.
+pub fn compose_system_prompt(names: &[&str], budget_chars: usize) -> String {
+    let mut parts: Vec<&str> = Vec::new();
+    for name in names {
+        if let Some(entry) = get_skill_by_name(name) {
+            parts.push(entry.content.as_str());
+        }
+    }
+    let mut out = String::new();
+    for part in parts {
+        let sep_len = if out.is_empty() { 0 } else { 2 };
+        if budget_chars != 0 && out.len() + sep_len + part.len() > budget_chars {
+            break;
+        }
+        if !out.is_empty() {
+            out.push_str("\n\n");
+        }
+        out.push_str(part);
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::loader::get_skill_by_name;
+
+    #[test]
+    fn composes_known_codegen_skills_in_order() {
+        let prompt = compose_system_prompt(&["codegen-planning"], 0);
+        assert!(!prompt.is_empty());
+        let direct = get_skill_by_name("codegen-planning").unwrap();
+        assert!(prompt.contains(direct.content.trim_end()));
+    }
+
+    #[test]
+    fn unknown_names_are_skipped() {
+        let only_known = compose_system_prompt(&["codegen-planning"], 0);
+        let with_bogus = compose_system_prompt(&["codegen-planning", "not-a-skill"], 0);
+        assert_eq!(only_known, with_bogus);
+    }
+
+    #[test]
+    fn budget_drops_trailing_skills_whole() {
+        let first = get_skill_by_name("codegen-chunk").unwrap();
+        let budget = first.content.len() + 1; // not enough for a second skill
+        let prompt = compose_system_prompt(&["codegen-chunk", "codegen-react"], budget);
+        assert_eq!(prompt, first.content);
+    }
+
+    #[test]
+    fn empty_names_yield_empty_prompt() {
+        assert_eq!(compose_system_prompt(&[], 0), "");
+    }
+}

crates/op-ai-skills/src/lib.rs

@@ -19,6 +19,7 @@
 use include_dir::{include_dir, Dir};
 
 pub mod budget;
+pub mod compose;
 pub mod frontmatter;
 pub mod loader;
 pub mod memory;
@@ -27,6 +28,7 @@ pub mod resolver;
 pub mod style_guide;
 pub mod types;
 
+pub use compose::compose_system_prompt;
 pub use loader::{get_skill_by_name, get_skill_registry, get_skills_by_phase, SkillEntry};
 pub use resolve::resolve_skills;
 pub use types::{