M5: skills (SKILL.md and skill tool)

Adds harness-core::config::markdown to parse SKILL.md frontmatter/body, a skill tool in harness-tools to invoke them, and system-prompt wiring so available skills are advertised to the model. Includes the rustfmt pass over the M5 files touched by this and the preceding LSP/rmcp work.

crates/harness-lsp/src/lib.rs

@@ -162,7 +162,10 @@ impl DiagnosticsSource for LspPool {
         let Some(client) = self.client_for(&server).await else {
             return;
         };
-        let ext = path.extension().and_then(|e| e.to_str()).unwrap_or_default();
+        let ext = path
+            .extension()
+            .and_then(|e| e.to_str())
+            .unwrap_or_default();
         let language_id = language_id_for_ext(ext);
 
         let existing_version = {

crates/harness-lsp/tests/rust_analyzer.rs

@@ -19,7 +19,11 @@ async fn rust_analyzer_reports_a_type_error() {
     .unwrap();
     std::fs::create_dir_all(dir.path().join("src")).unwrap();
     let main_rs = dir.path().join("src/main.rs");
-    std::fs::write(&main_rs, "fn main() {\n    let x: i32 = \"not an integer\";\n    let _ = x;\n}\n").unwrap();
+    std::fs::write(
+        &main_rs,
+        "fn main() {\n    let x: i32 = \"not an integer\";\n    let _ = x;\n}\n",
+    )
+    .unwrap();
 
     let pool = LspPool::new(dir.path().to_path_buf(), vec![]);
     pool.touch(&main_rs).await;