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.

Cargo.lock

@@ -711,7 +711,12 @@ dependencies = [
 name = "harness-lsp"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "harness-core",
+ "serde_json",
+ "tempfile",
+ "tokio",
+ "tracing",
 ]
 
 [[package]]