M5: LSP pool and diagnostics wiring

Adds the DiagnosticsSource seam trait in harness-core::lsp (kept out of harness-tools so tools never link the LSP crate directly), and implements it in harness-lsp as a pool that lazily spawns one language server per file extension (rust-analyzer, typescript-language-server, gopls, pyright-langserver) only when the binary is on PATH. Adds harness-tools::diagnostics and wires diagnostics into edit/write/bash/glob/grep output.

crates/harness-app/src/lib.rs

@@ -16,6 +16,7 @@ use harness_core::config::{self, Config, ConfigError};
 use harness_core::engine::jobs::{JobBoard, JobState, LaunchSpec};
 use harness_core::engine::{run_session, RunConfig, StepContext};
 use harness_core::event::{AppEvent, EventBus, RunOutcome};
+use harness_core::lsp::DiagnosticsSource;
 use harness_core::permission::{PermissionReply, PermissionService, Rule, Ruleset};
 use harness_core::store::{Store, StoreError};
 use harness_core::tool::{
@@ -90,6 +91,9 @@ struct EngineInner {
     providers: ProviderRegistry,
     catalog: ModelCatalog,
     agents: AgentRegistry,
+    /// LSP diagnostics pool (edit/write surface errors). Shared across sessions; servers spawn
+    /// lazily on first touch.
+    diagnostics: Option<Arc<dyn DiagnosticsSource>>,
     cwd: PathBuf,
     data_dir: PathBuf,
     runs: Mutex<HashMap<SessionId, RunHandle>>,
@@ -196,6 +200,20 @@ impl EngineHandle {
         // `init` warms the cache in the background for the next launch.
         let catalog = ModelCatalog::load_cached_or_baked(&ModelCatalog::default_cache_path());
 
+        // LSP pool: built-ins present on PATH, plus any config-declared servers.
+        let lsp_servers: Vec<harness_lsp::ServerConfig> = config
+            .lsp
+            .iter()
+            .map(|(name, c)| harness_lsp::ServerConfig {
+                name: name.clone(),
+                command: c.command.clone(),
+                args: c.args.clone(),
+                extensions: c.extensions.clone(),
+            })
+            .collect();
+        let diagnostics: Option<Arc<dyn DiagnosticsSource>> =
+            Some(Arc::new(harness_lsp::LspPool::new(cwd.clone(), lsp_servers)));
+
         let inner = Arc::new(EngineInner {
             config,
             store,
@@ -205,6 +223,7 @@ impl EngineHandle {
             providers,
             catalog,
             agents,
+            diagnostics,
             cwd,
             data_dir,
             runs: Mutex::new(HashMap::new()),
@@ -328,6 +347,7 @@ impl EngineHandle {
             spawner: Some(self.inner.clone()),
             job_board: Some(board),
             context_reporter: None, // root session has no parent board to report to
+            diagnostics: self.inner.diagnostics.clone(),
         };
         let run_config = RunConfig {
             agent_name: agent.name.clone(),
@@ -633,6 +653,7 @@ impl EngineInner {
             spawner: Some(self.arc()),
             job_board: Some(board),
             context_reporter: reporter,
+            diagnostics: self.diagnostics.clone(),
         }
     }
 }