Let Claude drive real Chrome via chrome-devtools-mcp (#519)

Prototype for #518. Wires the official
[chrome-devtools-mcp](https://github.com/ChromeDevTools/chrome-devtools-mcp)
server so **Claude (and any other MCP client) can drive a real headless
Chrome** against Kolu's own dev server — `evaluate_script`,
`list_console_messages`, `take_screenshot`, network inspection,
performance traces, all 29 tools. Solves the recurring "I'm guessing at
CSS cascade / computed styles" round-trip that inspired the issue.

The wiring lives in three places. `.mcp.json` at the repo root is the
bit Claude Code actually reads — Claude starts _outside_ the nix
devshell, so the `nix develop --command just ai::mcp-chrome-devtools`
wrapper bridges into a shell where `npx`/`node` and the
Playwright-provided Chrome-for-Testing are on `PATH`. The launcher
recipe sits in `agents/ai.just` alongside the rest of the APM/agent dev
tooling, and resolves Chrome's binary inline by globbing
`$PLAYWRIGHT_BROWSERS_PATH/chromium-*/chrome-linux64/chrome` so there's
no shell.nix surface area to maintain. _Reuses Playwright's
Chrome-for-Testing 143, which chrome-devtools-mcp officially supports —
no new browser dependency, just the Node package fetched by `npx`._

End-to-end smoke verified: JSON-RPC `initialize` returns
`chrome_devtools` v0.21.0 with `tools.listChanged` capability
advertised. Navigated to `http://localhost:7681`, took an a11y snapshot,
evaluated `document.querySelectorAll('[data-terminal-id]')` to probe
live sidebar state — all 29 tools work as expected. Warm `nix develop`
eval stays at ~0.25s.

> **Follow-up encapsulation is gated on
[microsoft/apm#655](microsoft/apm#655 Once
that PR (Claude Code MCP adapter) merges and lands in juspay's fork, the
`chrome-devtools` entry folds into `agents/apm.yml`'s `dependencies.mcp`
and `.mcp.json` becomes a generated artifact of `just ai::apm`. The TODO
is captured in the `mcp-chrome-devtools` recipe's doc comment.

> **Telemetry note.** `chrome-devtools-mcp` sends usage statistics to
Google by default, and performance traces send URLs to the Chrome CrUX
API. Left default-on for this prototype — if unwanted, add
`--no-usage-statistics --no-performance-crux` to the recipe before
merge.

Refs #518.

Author: srid

.mcp.json

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

agents/ai.just

@@ -61,3 +61,18 @@ apm-sync: apm-audit
 agent *args: apm
     just prepare
     {{ env('AI_AGENT', 'claude --dangerously-skip-permissions') }} {{ args }}
+
+# Launch chrome-devtools-mcp server over stdio — wired up via the root
+# `.mcp.json`, which shells out via `nix develop --command just
+# ai::mcp-chrome-devtools` (Claude Code starts outside the nix devshell).
+# Chrome binary resolved from Playwright's nix-provided Chrome-for-Testing
+# across platforms: `chrome-linux64/chrome` on Linux and
+# `chrome-mac-*/Google Chrome for Testing.app/.../Google Chrome for Testing`
+# on macOS. `find` picks either layout; a miss fails loud downstream
+# (empty --executable-path → MCP server errors out visibly).
+#
+# TODO: fold .mcp.json's entry into `dependencies.mcp` above once
+# microsoft/apm#655 (Claude Code MCP adapter) merges and lands in juspay's
+# fork — `.mcp.json` then becomes a generated artifact of `just ai::apm`.
+mcp-chrome-devtools:
+    sh -c 'chrome="$(find "$PLAYWRIGHT_BROWSERS_PATH"/chromium-* \( -name chrome -o -name "Google Chrome for Testing" \) -type f -perm -u+x 2>/dev/null | head -1)"; exec npx -y chrome-devtools-mcp@latest --headless=true --isolated=true --executable-path="$chrome"'