Add champion_strategies: what actually won your league, and why

Saves this session's league-winner analysis as a permanent tool instead
of scratch scripts. For each season, finds whichever team finished 1st
(ESPN's rankCalculatedFinal and pulls their real draft, reusing
sync_espn for name resolution. Every pick gets a value verdict --
preseason ECR against actual finish, the same steal/bust framing
value_picks and draft_backtest use -- so this answers what draft cost
actually paid off for the winner, not just what they drafted.

Reports each champion's opening two picks, first QB/TE round, RB/WR
volume, and biggest steal, plus cross-season aggregates (how often
champions opened RB-RB, median first-QB round). ECR history only goes
back to 2020; earlier seasons degrade gracefully to position/timing data
with null value verdicts rather than erroring.

Verified against a real 8-season league: numbers match the manual
analysis from earlier this session exactly (Matthew Stafford plus 90
spots in 2021, C.J. Stroud plus 74 in 2025), and pre-2020 seasons handle
the missing-ECR case cleanly.

Documented in README, docs/tools.md, and a new learning-from-history
section in docs/quickstart.md.

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