Longitudinal baseline history + drift trend (`arena trend`)

[macanderson]

Context. The regression gate compares one baseline to one run. To catch slow drift (not just today's pass/fail) Arena should keep a series.

Scope

• Append each accepted run's per-agent summary to arena-history.json.
• arena trend prints resolve-rate + median tokens/cost over time per agent.
• Feed the dashboard (#dashboard).

Acceptance. N runs produce a trend the gate and dashboard can both read.