feat: show judge variance from trend history

[igmarin]

Why

ROADMAP item 4 remainder: golden traces shipped, but live inter-run totals were not recorded. History already lives in .skill-bench-trends.json.

What

• TrendCalculator attaches judge_variance (n, mean, stddev, spread) from matching history + current context total
• Human table prints JUDGE n=… mean=… σ=… spread=… when n ≥ 2
• No --repeat flag, no verdict-math change

Proof

bundle exec ruby -Itest test/trend_tracker_test.rb
# 10 runs, 0 failures

bundle exec ruby -Itest test/services/delta_table_formatter_test.rb
# 9 runs, 0 failures

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 2d39e7d and 61fd5cb.

📒 Files selected for processing (6)

• CHANGELOG.md
• ROADMAP.md
• lib/skill_bench/services/delta_table_formatter.rb
• lib/skill_bench/trend_tracker/trend_calculator.rb
• test/services/delta_table_formatter_test.rb
• test/trend_tracker_test.rb

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