feat(mcp): wire chrome-devtools-mcp for Claude Code via nix+just

Add a Model Context Protocol server that gives Claude (and other MCP
clients) real Chrome-DevTools access — evaluate_script, console capture,
take_screenshot, network inspection, etc. — against Playwright's
nix-provided Chrome-for-Testing. Targets headless Linux.

Wiring:
- .mcp.json at repo root registers a "chrome-devtools" server that
  shells out via `nix develop --command just mcp-chrome-devtools`.
  Claude Code starts outside the devshell, so the nix-develop wrapper
  is mandatory — it bridges into the devshell where KOLU_CHROME_EXECUTABLE
  is set and npx/node are on PATH.
- justfile gains a `mcp-chrome-devtools` recipe that execs
  `npx -y chrome-devtools-mcp@latest` with --headless, --isolated, and
  --executable-path pinned to the resolved chrome binary. Keeps command
  complexity out of .mcp.json.
- shell.nix computes the chrome path at Nix eval time by reading
  playwright-driver's browsers.json (idiom from wiki.nixos.org/wiki/Playwright)
  — fails loud on layout change, warm `nix develop` eval stays at ~0.2s.

Telemetry: chrome-devtools-mcp sends usage stats to Google by default
(and trace URLs to Chrome CrUX for performance traces). Left on for the
prototype; add `--no-usage-statistics --no-performance-crux` to the just
recipe if that's unwanted.

Author: srid

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "chrome-devtools": {
+      "command": "nix",
+      "args": ["develop", "--command", "just", "mcp-chrome-devtools"]
+    }
+  }
+}

justfile

@@ -95,6 +95,13 @@ fmt:
 fmt-check:
     {{ nix_shell }} sh -c 'prettier --check --cache --ignore-unknown . && nixpkgs-fmt --check *.nix nix/**/*.nix'
 
+# Launch chrome-devtools-mcp server over stdio — wired up via .mcp.json.
+# Drives Playwright's nix-provided Chrome-for-Testing (headless, isolated
+# profile). Claude Code starts outside the devshell, so .mcp.json wraps
+# this with `nix develop --command just mcp-chrome-devtools`.
+mcp-chrome-devtools:
+    {{ nix_shell }} sh -c 'exec npx -y chrome-devtools-mcp@latest --headless=true --isolated=true --executable-path="$KOLU_CHROME_EXECUTABLE"'
+
 # Nix build (server + client)
 build:
     nix build

shell.nix

@@ -2,6 +2,17 @@
 { pkgs ? import ./nix/nixpkgs.nix { } }:
 let
   packages = import ./default.nix { inherit pkgs; };
+
+  # Chrome-for-Testing path for chrome-devtools-mcp (see .mcp.json). Pulled
+  # from playwright-driver's browsers.json at eval time so the chromium-N
+  # revision tracks whatever nixpkgs pins — fails loud here if layout changes
+  # instead of silently at MCP startup. Idiom from https://wiki.nixos.org/wiki/Playwright.
+  playwrightBrowsers = (builtins.fromJSON
+    (builtins.readFile "${pkgs.playwright-driver}/browsers.json")).browsers;
+  chromiumRev = (builtins.head
+    (builtins.filter (b: b.name == "chromium") playwrightBrowsers)).revision;
+  koluChromeExecutable =
+    "${pkgs.playwright-driver.browsers}/chromium-${chromiumRev}/chrome-linux64/chrome";
 in
 pkgs.mkShell {
   name = "kolu-shell";
@@ -11,6 +22,7 @@ pkgs.mkShell {
     KOLU_COMMIT_HASH = "dev";
     PLAYWRIGHT_BROWSERS_PATH = "${pkgs.playwright-driver.browsers}";
     PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD = "1";
+    KOLU_CHROME_EXECUTABLE = koluChromeExecutable;
   };
 
   shellHook = ''