feat(testing): add reusable Playwright browser journeys

[gbaudrit]

Summary

Adds the product-owned reusable Playwright foundation for #283 so Agentstration UX tests and communication captures share the same versioned journeys, explicit instance configuration, and campaign Workspace isolation. This PR is the baseline consumed by the dedicated codex/playwright-application-coverage integration branch and its child increments.

Changes

• Add the standalone Playwright workspace, isolated local product fixtures, capture CLI, stable Test ID/checkpoint/localization contracts, and reproducible capture manifests.
• Add reusable Console journeys for authentication and creating Agents, campaign Workspaces, handoff Flows, and Entries, with example capture plans.
• Support external Console and Workplace URLs, explicit credentials, installed browser channels, and campaign Workspace selection from the global Console header.
• Keep dependent journeys explicit: configured Workspaces and prerequisite resources are selected or diagnosed, never silently created or replaced.
• Document the automation and campaign isolation decisions in ADR-0109 and ADR-0110.
• Move browser validation into the independent browser-ux.yml workflow so the existing CI workflow remains unchanged.

Validation

• TypeScript compilation succeeded on the merged source with npm exec tsc -- --noEmit.
• npm audit --audit-level=high reported 0 vulnerabilities.
• CI, Documentation, Dependency Review, CodeQL, and pull-request metadata checks passed on the updated baseline.
• Browser UX exposed two post-main compatibility gaps on this baseline: the managed model fixture registration and generated Flow handoff routes. Both are fixed in test(playwright): enforce application route coverage catalog #297, whose four managed .NET/Chromium smoke journeys passed before it was squash-merged into codex/playwright-application-coverage.
• Local .NET tests remain unavailable because this execution environment does not provide the dotnet SDK.
• Mobile smoke remains tracked by Enforce cross-cutting browser quality gates #290.

Breaking changes

None.