E0: Repository, guardrails and work tracking

Phase P0 epic. See §7 of docs/UNIFIED-ARR-PLAN.md.

Exit criteria for this phase are defined in the plan and must be met before the next phase begins.

P0 exit criteria are met: PR #98 merged to protected main and run 30731081380 completed successfully across lint, test, build, both UI jobs, UI E2E, and container. Repository governance, documentation, issue/project tracking, dependency guardrails, and hosted CI foundation are in place.