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-lsp/Cargo.toml

@@ -6,6 +6,13 @@ license.workspace = true
 
 [dependencies]
 harness-core = { workspace = true }
+tokio = { workspace = true }
+async-trait = { workspace = true }
+serde_json = { workspace = true }
+tracing = { workspace = true }
+
+[dev-dependencies]
+tempfile = { workspace = true }
 
 [lints]
 workspace = true