feat(label): add king:defeat label

[carlos4s]

Summary

Tunes the Autovara/kata entry in master_repositories.json so a newly crowned king clearly out-earns the king it replaced, and rewards fade over a shorter window.

Changes

• Add kata:defeat: 0.25 — when a new king is promoted, the previous king's PR is relabeled kata:defeat. At 0.25 it sits below the smallest reward tier (kata:reward:s = 1.5), so a dethroned king always earns less than any current king (winner-take-all, with a small residual instead of zero).
• Remove dead kata:evaluating — Kata never emits this label, and at 0.0 it duplicated default_label_multiplier: 0.0. Removing it also keeps the entry within the 10-label limit enforced by the live-config test.
• pr_lookback_days: 14 → 7 — shorter reward memory.
• Time decay retuned for the 7-day window — sigmoid_midpoint_days 2 → 3, sigmoid_steepness 1.0 → 0.75. Gentle curve that reaches the 5% floor right at the 7-day edge (no cliff): ~0.90 (day 0) → 0.50 (day 3) → 0.05 (day 7).

Notes

• Kata's emission_share is unchanged; total shares still sum to 1.0.
• The kata:defeat multiplier only takes effect once the Kata bot strips the ex-king's kata:winner:* / kata:reward:* labels (label scoring uses the max of matching labels). That change ships separately in the Kata repo.

Testing

• master_repositories.json parses and loads via load_master_repo_weights.
• Full validator/config suite passes (emission-share, eligibility, scoring, and the label-multiplier bound test).