fix(security): isolate production browser walks

[FranciscoMateusVG]

Summary

• run the shared Mac Mini playwright-mini launcher in Playwright MCP isolated mode
• require a null-session preflight before every production walk
• fail closed on inherited sessions and require sign-out/null-session/browser-close cleanup
• limit walk authentication to the allowlisted BEADS identity; customer accounts require explicit operator authorization
• record identity metadata only, never browser credentials or session material

Runtime contract changes

• Playwright Mini profile lifetime — production browser state is now in-memory and discarded when the browser closes instead of persisting in the shared workspace profile.
  • Category: authentication/session infrastructure
  • Consumers affected: agents and subagents using playwright-mini for production verification
  • Migration plan: none; each walk must authenticate its declared allowlisted identity after the mandatory null-session preflight
• Production-walk session gate — a walk cannot proceed unless /api/auth/get-session is null; unexpected sessions are signed out and rechecked, and cleanup repeats the null-session assertion.
  • Category: authentication gate
  • Consumers affected: verify-user-path production walks
  • Migration plan: BEADS walk notes record metadata-only preflight/final results

Security finding

aperture-4iz7v traced a real customer's authenticated session from a 2026-07-23 verification into the shared persistent browser profile, where a different agent inherited it on 2026-07-27. The session was contained by sign-out. This patch removes the persistence mechanism and codifies fail-closed lifecycle checks.

Validation

• bash -n scripts/playwright-mcp-mini.sh
• remote playwright-mcp --help confirms --isolated is supported
• static assertions confirm the launcher flag and every required verify-user-path control
• git diff --check

Base branch note

This repository's canonical branch is master (origin/HEAD -> origin/master); origin/main does not exist, so the PR targets master.