feat: sync agent-browser skill with upstream vercel-labs/agent-browser

Update SKILL.md to match the latest upstream skill from
vercel-labs/agent-browser, adding substantial new capabilities:

- Authentication (auth vault, profiles, session persistence, state files)
- Command chaining, annotated screenshots, diffing
- Security features (content boundaries, domain allowlist, action policy)
- iOS Simulator support, Lightpanda engine, downloads, clipboard
- JS eval improvements (--stdin, -b for shell safety)
- Timeout guidance, config files, session cleanup

Add 7 reference docs (commands, authentication, snapshot-refs,
session-management, video-recording, profiling, proxy-support) and
3 ready-to-use shell templates.

Kept our YAML frontmatter, setup check section, and Playwright MCP
comparison table which are unique to our plugin context.

plugins/compound-engineering/skills/agent-browser/references/profiling.md

@@ -0,0 +1,120 @@
+# Profiling
+
+Capture Chrome DevTools performance profiles during browser automation for performance analysis.
+
+**Related**: [commands.md](commands.md) for full command reference, [SKILL.md](../SKILL.md) for quick start.
+
+## Contents
+
+- [Basic Profiling](#basic-profiling)
+- [Profiler Commands](#profiler-commands)
+- [Categories](#categories)
+- [Use Cases](#use-cases)
+- [Output Format](#output-format)
+- [Viewing Profiles](#viewing-profiles)
+- [Limitations](#limitations)
+
+## Basic Profiling
+
+```bash
+# Start profiling
+agent-browser profiler start
+
+# Perform actions
+agent-browser navigate https://example.com
+agent-browser click "#button"
+agent-browser wait 1000
+
+# Stop and save
+agent-browser profiler stop ./trace.json
+```
+
+## Profiler Commands
+
+```bash
+# Start profiling with default categories
+agent-browser profiler start
+
+# Start with custom trace categories
+agent-browser profiler start --categories "devtools.timeline,v8.execute,blink.user_timing"
+
+# Stop profiling and save to file
+agent-browser profiler stop ./trace.json
+```
+
+## Categories
+
+The `--categories` flag accepts a comma-separated list of Chrome trace categories. Default categories include:
+
+- `devtools.timeline` -- standard DevTools performance traces
+- `v8.execute` -- time spent running JavaScript
+- `blink` -- renderer events
+- `blink.user_timing` -- `performance.mark()` / `performance.measure()` calls
+- `latencyInfo` -- input-to-latency tracking
+- `renderer.scheduler` -- task scheduling and execution
+- `toplevel` -- broad-spectrum basic events
+
+Several `disabled-by-default-*` categories are also included for detailed timeline, call stack, and V8 CPU profiling data.
+
+## Use Cases
+
+### Diagnosing Slow Page Loads
+
+```bash
+agent-browser profiler start
+agent-browser navigate https://app.example.com
+agent-browser wait --load networkidle
+agent-browser profiler stop ./page-load-profile.json
+```
+
+### Profiling User Interactions
+
+```bash
+agent-browser navigate https://app.example.com
+agent-browser profiler start
+agent-browser click "#submit"
+agent-browser wait 2000
+agent-browser profiler stop ./interaction-profile.json
+```
+
+### CI Performance Regression Checks
+
+```bash
+#!/bin/bash
+agent-browser profiler start
+agent-browser navigate https://app.example.com
+agent-browser wait --load networkidle
+agent-browser profiler stop "./profiles/build-${BUILD_ID}.json"
+```
+
+## Output Format
+
+The output is a JSON file in Chrome Trace Event format:
+
+```json
+{
+  "traceEvents": [
+    { "cat": "devtools.timeline", "name": "RunTask", "ph": "X", "ts": 12345, "dur": 100 },
+    ...
+  ],
+  "metadata": {
+    "clock-domain": "LINUX_CLOCK_MONOTONIC"
+  }
+}
+```
+
+The `metadata.clock-domain` field is set based on the host platform (Linux or macOS). On Windows it is omitted.
+
+## Viewing Profiles
+
+Load the output JSON file in any of these tools:
+
+- **Chrome DevTools**: Performance panel > Load profile (Ctrl+Shift+I > Performance)
+- **Perfetto UI**: https://ui.perfetto.dev/ -- drag and drop the JSON file
+- **Trace Viewer**: `chrome://tracing` in any Chromium browser
+
+## Limitations
+
+- Only works with Chromium-based browsers (Chrome, Edge). Not supported on Firefox or WebKit.
+- Trace data accumulates in memory while profiling is active (capped at 5 million events). Stop profiling promptly after the area of interest.
+- Data collection on stop has a 30-second timeout. If the browser is unresponsive, the stop command may fail.