Live: lock down the local server against same-machine token theft (#304)

Two defense-in-depth layers close the P1 in issue #304, where any browser
tab on the machine could fetch /live.js, extract the embedded token, and
drive every token-gated route.

1. Loopback-restricted CORS. The shared handler replaced its wildcard
   `Access-Control-Allow-Origin: *` with reflection gated on a strict
   isLoopbackOrigin() that URL-parses the Origin (so localhost.evil.com and
   127.0.0.1.evil.com fail) and accepts only http/https on localhost,
   127.0.0.1, or [::1]. Reflection always pairs with `Vary: Origin` so a
   cache never hands one origin's authorized response to another. Remote
   origins get no ACAO header; origin-less callers (script tags, curl, the
   agent's own fetches) are unaffected.

2. Token-gated /live.js. The handler now 401s unless `?token=` matches
   state.token, so the bundle (which embeds the token) is no longer served
   to unauthenticated local pages. The injected <script src> carries the
   token: live.mjs passes --token to live-inject.mjs, which threads it
   through every injection path (HTML/JSX tag, Nuxt plugin, SvelteKit root
   component) via a shared buildLiveScriptSrc(). The token stays optional in
   live-inject so static fixture tests keep their bare src.

Tests: new live-server integration cases for the 401 gate, remote-origin
denial, loopback reflection + Vary, and token-guarded routes under a
loopback Origin; e2e session harness now injects with the token.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skill/scripts/live.mjs

@@ -112,7 +112,11 @@ The agent should then:
   }
 
   // 3. Inject the script tag at the current port
-  const injectOut = runScript('live-inject.mjs', ['--port', String(serverInfo.port)], { cwd: activeCwd });
+  const injectOut = runScript(
+    'live-inject.mjs',
+    ['--port', String(serverInfo.port), '--token', String(serverInfo.token)],
+    { cwd: activeCwd },
+  );
   const injectResult = safeParse(injectOut);
   if (!injectResult || !injectResult.ok) {
     console.log(JSON.stringify({