cookie-import-browser should clear localStorage/sessionStorage before importing

## Symptom

After a Clerk SDK major upgrade (v5 → v7 in our case), `/setup-browser-cookies` imports fresh cookies but the browse session still enters an infinite redirect loop: `Refreshing the session token resulted in an infinite redirect loop`. Recovery requires clearing the browse session's localStorage manually.

## Root cause

Clerk stores a `clerk_environment` entry in localStorage that includes the SDK major version (`clerk_js_version: "5"`) and an `exp` timestamp. After upgrading to v7 server-side, the browse browser's localStorage still has the v5-shaped entry with an expired `exp`. Clerk's client JS reads the stale entry first and fails to reconcile with the fresh v7 server state.

`cookie-import-browser` touches cookies only — localStorage survives. Every re-import hits the same stale state.

Reference: [clerk/javascript#1436](https://github.com/clerk/javascript/issues/1436) — same failure mode, same fix (clear local/session storage).

## Proposed change

In `cookie-import-browser`, before applying imported cookies to a given origin, also clear its localStorage and sessionStorage:

\`\`\`ts
// Pseudocode — wherever cookies are applied per origin
await page.goto(origin);
await page.evaluate(() => {
localStorage.clear();
sessionStorage.clear();
});
await context.addCookies(importedCookiesForOrigin);
\`\`\`

Behind an opt-out flag if there's concern about breaking flows that depend on pre-existing localStorage:

\`\`\`
cookie-import-browser [browser] [--domain ] [--keep-storage]
\`\`\`

Default should be "clear" — the stated purpose of the command is to bring the browse session into the same authenticated state as the real browser, and for any modern auth SDK (Clerk, Auth0, Firebase, Supabase) that state lives across cookies **and** localStorage.

## Risk

Low. Anyone currently relying on pre-existing localStorage in the browse session is doing so implicitly — there's no documented contract that `cookie-import-browser` preserves it. The `--keep-storage` escape hatch covers the edge case.

## Acceptance

Running `cookie-import-browser` against a site that stores auth metadata in localStorage (Clerk, Auth0, etc.) leaves the browse session in a consistent state with the real browser, with no stale version entries.

## Context

Origin: B81 / SIX-132 in our internal tracker. Keel-side workaround lives in a wrapper skill (`/auth-reset`) + shell script that navigates to the origin and clears storage via `browse js`. Would love to retire that once this lands upstream.