Cover OpenCode refresh session recovery

[danshapiro]

Summary

• Add browser-level regression coverage for UI-created OpenCode panes surviving refresh.
• Add a fake-opencode session-event gate so tests can prove hidden sessionRef reconciliation when association arrives while the browser is closed.
• Keep the implementation plan in docs/superpowers/plans for the red/green proof trail.

Red/Green Evidence

• Red proof on pre-client-reconciliation base 8e1492b4: hidden sessionRef E2E failed at restore-page wait after the server-side sessionRef existed.
• Green: timeout 420s npm run test:e2e:chromium -- test/e2e-browser/specs/opencode-restart-recovery.spec.ts -g "recovers a hidden OpenCode sessionRef when association lands while the browser is closed" passed.
• Green: timeout 480s npm run test:e2e:chromium -- test/e2e-browser/specs/opencode-restart-recovery.spec.ts passed, 5 tests.
• Green: timeout 240s npm run test:vitest -- test/unit/client/components/App.ws-bootstrap.test.tsx test/unit/client/components/TerminalView.resumeSession.test.tsx --run passed, 37 tests.
• Clean: git diff --check origin/main..HEAD.

Review

• Fresheyes pass 1 found nits; fixed.
• Fresheyes pass 2 passed with only non-actionable nits.