test(mcp): agent smoke test across five coding agents, tool snapshot

- tests/agents/agent-smoke.sh runs tests/agents/cases.json through
  Claude Code, Codex, pi, Hermes Agent, and opencode headless against a
  local idx-mcp build, each with an isolated config, and checks the
  tools called, their key arguments, and (mock data) facts in the
  answer. Opt-in; never part of the gate.
- crates/idx-mcp/tests/golden/tools.json pins tool names, descriptions,
  and input schemas, so changes to what agents see are reviewed.
- nix develop .#agents adds Claude Code, Codex, and opencode from
  nixpkgs after the host PATH.

First run: 30/30 (5 agents x 6 cases, mock data).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

flake.nix

@@ -280,6 +280,7 @@
             shellcheck -s sh ${installerSrc}/install.sh
             shellcheck ${installerSrc}/scripts/install-sh-test.sh
             shellcheck ${./scripts/live-smoke.sh}
+            shellcheck ${./tests/agents/agent-smoke.sh}
             touch "$out"
           '';
           publisher-test = pkgs.runCommand "idx-publisher-test" {
@@ -296,6 +297,23 @@
           '';
         };
 
+        # `nix develop .#agents`: the default shell plus coding-agent CLIs for
+        # tests/agents/agent-smoke.sh. They are appended to PATH, so agents
+        # already installed (and logged in) on the host take precedence.
+        # pi and Hermes Agent are not in the pinned nixpkgs; use host installs.
+        devShells.agents =
+          let
+            agentPkgs = import nixpkgs {
+              inherit system;
+              config.allowUnfreePredicate = pkg: lib.getName pkg == "claude-code";
+            };
+          in
+          self.devShells.${system}.default.overrideAttrs (old: {
+            shellHook = (old.shellHook or "") + ''
+              export PATH="$PATH:${lib.makeBinPath (with agentPkgs; [ claude-code codex opencode ])}"
+            '';
+          });
+
         devShells.default = pkgs.mkShell {
           inputsFrom = [ idxPackage ];
           packages = with pkgs; [