Explain biggest_steal, not just report it, in champion_strategies

Adds a context block to each season's biggest_steal: usage_trend (real
early- vs late-season carries/targets/target share, or pass attempts
and yards for a QB) and team_environment (O-line ranks, pace, pass/rush
split for that specific season). This is the concrete why behind a value
pick instead of just the fact that it happened -- most steals turn out
to be a role, volume, or situational story rather than raw talent
beating a forecast, and this makes that visible in the actual box
scores rather than leaving it to speculation.

QB dominates biggest_steal in this league's history, so usage_trend also
tracks pass attempts and yards, not just carries/targets, since those
alone read as near zero for a passer and said nothing useful. Verified
against a real season: pass attempts actually fell in the second half as
the team leaned more on the run game, a more accurate and more specific
story than a generic role-expansion narrative would have suggested.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>