feat(react): React introspection, Web Vitals, and SPA primitives (#1257)

* feat(react): first-class React introspection, Web Vitals, and nextjs skill

Add React-general and web-universal features as first-class agent-browser verbs
(react tree/inspect/renders/suspense, vitals, pushstate). Genuinely Next.js-specific
workflows (PPR cookie protocol, /_next/mcp bridge, dev-server endpoints) ship as
a new `nextjs` skill that composes the primitives. No new runtime dependencies -
the React DevTools installHook.js is vendored (MIT) and include_str!'d into the
binary.

New commands:
  react tree                  Full React component tree (depth id parent name)
  react inspect <fiberId>     Props, hooks, state, source for one fiber
  react renders start|stop    Fiber profiler with Insts/Mounts/Re-renders/Self/DOM
                              + prev->next change details
  react suspense              Suspense boundaries + classifier (client-hook,
                              request-api, server-fetch, cache, stream, framework)
                              + root-cause grouping + recommendations
  vitals [url]                LCP/CLS/TTFB/FCP/INP + React hydration phases
  pushstate <url>             Generic SPA client-side navigation
  removeinitscript <id>       Remove a script registered via addinitscript

New launch flags:
  --init-script <path>        Register init scripts before first navigation
                              (repeatable; env AGENT_BROWSER_INIT_SCRIPTS)
  --enable <feature>          Built-in init scripts; currently react-devtools
                              (repeatable; env AGENT_BROWSER_ENABLE)

Other primitives:
  network route ... --resource-type <csv>  Filter by CDP resource type
  cookies set --curl <file>                Auto-detects JSON/cURL/Cookie-header

* fixes

* fixes

* fixes

docs/src/app/commands/page.mdx

@@ -3,7 +3,8 @@
 ## Core
 
 ```bash
-agent-browser open <url>              # Navigate (aliases: goto, navigate)
+agent-browser open                    # Launch browser (no nav); stays on about:blank
+agent-browser open <url>              # Launch + navigate (aliases: goto, navigate)
 agent-browser click <sel>             # Click element (--new-tab to open in new tab)
 agent-browser dblclick <sel>          # Double-click
 agent-browser fill <sel> <text>       # Clear and fill
@@ -173,6 +174,7 @@ agent-browser storage session         # Same for sessionStorage
 agent-browser network route <url>              # Intercept requests
 agent-browser network route <url> --abort      # Block requests
 agent-browser network route <url> --body <json>  # Mock response
+agent-browser network route '*' --abort --resource-type script  # Block scripts only
 agent-browser network unroute [url]            # Remove routes
 agent-browser network requests                 # View tracked requests
 agent-browser network requests --clear         # Clear request log
@@ -420,6 +422,51 @@ Chat-specific options:
 agent-browser back                    # Go back
 agent-browser forward                 # Go forward
 agent-browser reload                  # Reload page
+agent-browser pushstate <url>         # SPA client-side nav; auto-detects window.next.router.push,
+                                      # falls back to history.pushState + popstate
+```
+
+## Pre-navigation setup
+
+Some flows need routes, cookies, or init scripts configured *before* the
+first navigation (SSR debug, auth on protected origins, etc.). `open`
+without a URL launches the browser but stays on `about:blank`, leaving
+room to stage state. `batch` makes it one CLI invocation:
+
+```bash
+agent-browser batch \
+  '["open"]' \
+  '["network","route","*","--abort","--resource-type","script"]' \
+  '["cookies","set","--curl","cookies.curl","--domain","localhost"]' \
+  '["navigate","http://localhost:3000/target"]'
+```
+
+## React / Web Vitals
+
+React commands require `--enable react-devtools` at launch (installs the
+React DevTools hook before any page JS runs). `vitals` and `pushstate`
+work on any site.
+
+```bash
+agent-browser open --enable react-devtools <url>   # Launch with React hook installed
+agent-browser react tree                           # Full component tree
+agent-browser react inspect <fiberId>              # Inspect one component
+agent-browser react renders start                  # Begin fiber render recording
+agent-browser react renders stop [--json]          # Stop + print profile
+agent-browser react suspense [--only-dynamic] [--json]  # Suspense boundaries + classifier
+                                                         # --only-dynamic hides the "static" list
+agent-browser vitals [url] [--json]                # LCP/CLS/TTFB/FCP/INP + hydration
+```
+
+Works on any React app (Next.js, Remix, Vite+React, CRA, TanStack Start,
+React Native Web, etc.). `vitals` and `pushstate` are framework-agnostic.
+
+## Init scripts
+
+```bash
+agent-browser open --init-script <path>           # Register before first navigation (repeatable)
+agent-browser addinitscript <js>                  # Register at runtime (returns identifier)
+agent-browser removeinitscript <identifier>       # Remove a previously registered init script
 ```
 
 ## Global options
@@ -432,6 +479,8 @@ agent-browser reload                  # Reload page
 --headers <json>         # HTTP headers scoped to URL's origin
 --executable-path <path> # Custom browser executable
 --extension <path>       # Load browser extension (repeatable)
+--init-script <path>     # Register a page init script before first navigation (repeatable)
+--enable <feature>       # Built-in init scripts: react-devtools (repeatable or comma-list)
 --args <args>            # Browser launch args (comma separated)
 --user-agent <ua>        # Custom User-Agent string
 --proxy <url>            # Proxy server URL