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-core/src/engine/loop.rs

@@ -434,6 +434,7 @@ mod tests {
             spawner: None,
             job_board: None,
             context_reporter: None,
+            diagnostics: None,
         }
     }
 

crates/harness-core/src/engine/processor.rs

@@ -72,6 +72,8 @@ pub struct StepContext {
     /// Present in subagent sessions: reports read files to this session's job on the parent
     /// board. `None` for root sessions (nothing to report to).
     pub context_reporter: Option<Arc<dyn ContextReporter>>,
+    /// Language-server diagnostics source shared by the session's edit/write tool calls.
+    pub diagnostics: Option<Arc<dyn crate::lsp::DiagnosticsSource>>,
 }
 
 struct FlushTracker {
@@ -522,6 +524,7 @@ impl<'a> Run<'a> {
             metadata: metadata_sink,
             spawner: self.ctx.spawner.clone(),
             context_reporter: self.ctx.context_reporter.clone(),
+            diagnostics: self.ctx.diagnostics.clone(),
         };
 
         let result = tokio::select! {

crates/harness-core/src/lib.rs

@@ -3,6 +3,7 @@ pub mod config;
 pub mod engine;
 pub mod event;
 pub mod llm;
+pub mod lsp;
 pub mod permission;
 pub mod store;
 pub mod tool;

crates/harness-core/src/lsp.rs

@@ -0,0 +1,46 @@
+//! Seam trait for language-server diagnostics, implemented by `harness-lsp` and consumed by
+//! the edit/write tools. Kept in core so `harness-tools` never links the LSP crate directly.
+//! See `docs/09-integrations.md`.
+
+use std::path::Path;
+use std::time::Duration;
+
+use async_trait::async_trait;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Severity {
+    Error,
+    Warning,
+    Info,
+    Hint,
+}
+
+/// One diagnostic reported by a language server. Line/character are 1-based for display.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Diagnostic {
+    pub line: u32,
+    pub character: u32,
+    pub severity: Severity,
+    pub message: String,
+    pub source: Option<String>,
+}
+
+impl Diagnostic {
+    /// `{file}: L{line}: {message}` — the one-line form appended to tool output.
+    pub fn display_line(&self, file: &str) -> String {
+        format!("{file}: L{}: {}", self.line, self.message)
+    }
+}
+
+/// A source of file diagnostics (an LSP pool). All methods are best-effort: failures are
+/// swallowed (logged) so a broken language server never fails a tool call.
+#[async_trait]
+pub trait DiagnosticsSource: Send + Sync {
+    /// Ensure a server for `path`'s language is running and told about the file's current
+    /// contents (spawn-if-needed + didOpen/didChange).
+    async fn touch(&self, path: &Path);
+
+    /// Diagnostics for `path`, waiting up to `wait` for the server to (re)publish after a
+    /// change. Returns whatever is known on timeout.
+    async fn diagnostics(&self, path: &Path, wait: Duration) -> Vec<Diagnostic>;
+}

crates/harness-core/src/tool/mod.rs

@@ -195,6 +195,9 @@ pub struct ToolCtx {
     pub spawner: Option<Arc<dyn SubagentSpawner>>,
     /// Present in subagent sessions: lets the read tool report files to the job board.
     pub context_reporter: Option<Arc<dyn ContextReporter>>,
+    /// Language-server diagnostics source (edit/write surface errors after a change). `None`
+    /// disables LSP integration.
+    pub diagnostics: Option<Arc<dyn crate::lsp::DiagnosticsSource>>,
 }
 
 #[derive(Debug)]