fix: preserve browser sessions across action failures

[iamjr15]

Summary

• binds each deterministic browser action to the driver's retained observation in one serialized request
• removes the separate Worker-to-driver state request that doubled preparation and could race a restart
• preserves a healthy Chromium session after an ordinary click/fill failure; only security or runtime-integrity failures discard it

Architecture

The sandbox driver now owns the full observation boundary: element ref, exact page URL, XPath, and allowed origin. The Worker sends only the bounded method/ref/value. The driver derives and enforces URL/origin guards immediately before replaying the observed Stagehand action, then records the new tree atomically.

Decisions Made

Decision	Choice	Reasoning
Security binding owner	Sandbox driver	The retained observation is authoritative and cannot race a separate state call
Ordinary action failure	Keep browser, invalidate refs	A click timeout does not prove browser integrity was lost
Security/runtime failure	Discard runtime	Origin interception or active-page integrity failures require a clean browser

Verification

• pnpm lint
• pnpm typecheck
• pnpm turbo build --force
• pnpm deadcode
• pnpm architecture:check
• pnpm turbo skills:build
• node --check infra/containers/sandbox/browser-driver/server.js

Production browser QA will follow immutable sandbox snapshot publication and promotion because the driver is image-owned.