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-app/src/lib.rs

@@ -159,6 +159,9 @@ struct EngineInner {
     /// LSP diagnostics pool (edit/write surface errors). Shared across sessions; servers spawn
     /// lazily on first touch.
     diagnostics: Option<Arc<dyn DiagnosticsSource>>,
+    /// Pre-rendered "## Skills" system block (name + description), injected into every run.
+    /// `None` when no skills are configured.
+    skills_prompt: Option<String>,
     cwd: PathBuf,
     data_dir: PathBuf,
     runs: Mutex<HashMap<SessionId, RunHandle>>,
@@ -217,6 +220,14 @@ impl EngineHandle {
             tools.register(tool);
         }
 
+        // Skills: layered global + project markdown, advertised in the system prompt and pulled
+        // on demand by the `skill` tool (only registered when at least one skill exists).
+        let global_skill_dir = dirs::config_dir().map(|d| d.join("ai-harness").join("skill"));
+        let project_skill_dir = cwd.join(".harness").join("skill");
+        let skills = config::load_skills(global_skill_dir.as_deref(), Some(&project_skill_dir));
+        harness_tools::register_skill_tool(&mut tools, &skills);
+        let skills_prompt = config::skills_prompt(&skills);
+
         // Layered agent registry: bundled markdown → global dir → project dir → config patches.
         let global_agent_dir = dirs::config_dir().map(|d| d.join("ai-harness").join("agent"));
         let project_agent_dir = cwd.join(".harness").join("agent");
@@ -259,6 +270,7 @@ impl EngineHandle {
             catalog,
             agents,
             diagnostics,
+            skills_prompt,
             cwd,
             data_dir,
             runs: Mutex::new(HashMap::new()),
@@ -395,6 +407,7 @@ impl EngineHandle {
             inject_job_board,
             reminder_turn_start: self.inner.reminder("turn_start"),
             reminder_after_file_tool: self.inner.reminder("after_file_tool"),
+            skills_prompt: self.inner.skills_prompt.clone(),
         };
 
         // Reserve the run slot *before* spawning. If we inserted after spawning, a run that
@@ -807,6 +820,7 @@ impl SubagentSpawner for EngineInner {
             inject_job_board: agent.mode.is_primary(),
             reminder_turn_start: self.reminder("turn_start"),
             reminder_after_file_tool: self.reminder("after_file_tool"),
+            skills_prompt: self.skills_prompt.clone(),
         };
 
         // Register the launch on the parent board (also makes foreground results reusable).
@@ -1321,6 +1335,25 @@ mod tests {
         assert_eq!(app.engine.inner.tools.all().len(), 7);
     }
 
+    #[tokio::test]
+    async fn project_skill_registers_the_skill_tool_and_advertises_it() {
+        let dir = tempfile::tempdir().unwrap();
+        let skill_dir = dir.path().join(".harness").join("skill").join("fmt");
+        std::fs::create_dir_all(&skill_dir).unwrap();
+        std::fs::write(
+            skill_dir.join("SKILL.md"),
+            "---\ndescription: format the code\n---\nRun cargo fmt.",
+        )
+        .unwrap();
+
+        let engine = EngineHandle::init_in_memory(dir.path().to_path_buf()).unwrap();
+        // 6 built-ins + task + skill.
+        assert_eq!(engine.inner.tools.all().len(), 8);
+        assert!(engine.inner.tools.get("skill").is_some());
+        let prompt = engine.inner.skills_prompt.clone().expect("skills advertised");
+        assert!(prompt.contains("**fmt** — format the code"));
+    }
+
     #[tokio::test]
     async fn run_prompt_errors_on_unregistered_provider() {
         let dir = tempfile::tempdir().unwrap();