HTML report output (`arena report --format html`)

**Context.** `arena report` emits markdown; a self-contained HTML report is easier to share with non-engineers.

**Scope.** `arena report <runDir> --format html` → a single HTML file (per-agent table, per-task matrix, CIs, reproduce command) with no external assets.

**Acceptance.** Opens offline; contains the same numbers as `report.md`.
