refactor(browser): replace workspace with explicit sessions

[jackwener]

Summary

• replace the browser-facing --workspace model with required --session <name> on opencli browser *
• remove bind guards/options (--domain, --path-prefix, --allow-navigate-bound) and remove browser namespace --keep-tab
• route extension leases through structured metadata: session, surface, kind, with browser result/session docs updated
• keep adapter browserSession.reuse: 'site' behavior unchanged; only the browser user surface changes here

Verification

• npx tsc --noEmit
• npm test -- --run clis/notebooklm/*.test.js src/cli.test.ts src/browser/page.test.ts src/browser/daemon-client.test.ts src/browser/network-cache.test.ts src/doctor.test.ts src/execution.test.ts src/observation/manager.test.ts src/observation/artifact.test.ts
• npx vitest run --project extension extension/src/background.test.ts
• npm run build
• npm run typecheck && npm run build in extension/
• npm run docs:build
• npm run check:typed-error-lint
• npm run check:silent-column-drop
• git diff --check

Notes

Breaking by design: no backward compatibility for --workspace/bound: browser surface. Adapter site-continuity rename to siteSession: 'persistent' is a follow-up PR.