feat(coding-agent): add Expert Elixir language server to built-in LSP defaults

Adds an `expert` server entry (binary `expert`, invoked `expert --stdio`)
to the built-in LSP defaults, auto-detected for Mix projects
(`mix.exs`/`mix.lock`). Placed after `elixirls` so elixir-ls remains the
primary navigation server when both binaries are installed; this keeps the
default getServerForFile() behavior unchanged. Absent-binary setups are
unaffected by loadConfig's PATH/root-marker gating.

Includes a docs row, changelog entry, and a regression test pinning that
`.ex` files match `expert` and that `elixirls` still sorts ahead of it.

docs/lsp-config.md

@@ -211,6 +211,7 @@ The following servers ship in `defaults.json` and are eligible for auto-detectio
 | `hls`                         | Haskell                       | `haskell-language-server-wrapper` |
 | `ocamllsp`                    | OCaml                         | `ocamllsp`                        |
 | `elixirls`                    | Elixir                        | `elixir-ls`                       |
+| `expert`                      | Elixir                        | `expert`                          |
 | `erlangls`                    | Erlang                        | `erlang_ls`                       |
 | `gleam`                       | Gleam                         | `gleam`                           |
 | `solargraph`                  | Ruby                          | `solargraph`                      |

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added the Expert Elixir language server (`expert`, invoked as `expert --stdio`) to the built-in LSP server list, auto-detected for Mix projects (`mix.exs`/`mix.lock`). When both are installed, `elixir-ls` remains the primary navigation server (Expert is ordered after it).
+
 ## [15.11.0] - 2026-06-10
 
 ### Breaking Changes

packages/coding-agent/src/lsp/defaults.json

@@ -248,6 +248,12 @@
 			}
 		}
 	},
+	"expert": {
+		"command": "expert",
+		"args": ["--stdio"],
+		"fileTypes": [".ex", ".exs", ".heex", ".eex"],
+		"rootMarkers": ["mix.exs", "mix.lock"]
+	},
 	"erlangls": {
 		"command": "erlang_ls",
 		"args": [],

packages/coding-agent/test/tools/lsp-regressions.test.ts

@@ -38,6 +38,7 @@ import type { ToolSession } from "@oh-my-pi/pi-coding-agent/tools";
 import { clampTimeout } from "@oh-my-pi/pi-coding-agent/tools/tool-timeouts";
 import * as piUtils from "@oh-my-pi/pi-utils";
 import { sanitizeText, TempDir } from "@oh-my-pi/pi-utils";
+import DEFAULTS from "../../src/lsp/defaults.json" with { type: "json" };
 
 describe("lsp regressions", () => {
 	afterEach(() => {
@@ -1829,3 +1830,13 @@ for await (const chunk of Bun.stdin.stream()) {
 		expect(output).toContain("typescript-language-server (ready)");
 	});
 });
+
+describe("expert elixir lsp", () => {
+	it("registers expert for .ex while keeping elixirls primary", () => {
+		const config = { servers: DEFAULTS as unknown as Record<string, ServerConfig> };
+		const names = getServersForFile(config, "lib/app.ex").map(([name]) => name);
+		expect(names).toContain("expert");
+		expect(names).toContain("elixirls");
+		expect(names.indexOf("elixirls")).toBeLessThan(names.indexOf("expert"));
+	});
+});