Good Egg v3: simplify scoring to merge_rate only for unknown contributors

[jeffreyksmithjr]

Summary

Simplify the GE scoring formula for unknown contributors (the population scored when skip_known_contributors=true). Drop hub_score and log_account_age from the v2 LR; use alltime merge_rate as the sole scoring input.

Evidence

Experiments on the bot-detection branch (PR #44), tracked in experiments/bot_detection/RESULTS.md:

hub_score hurts unknown contributors (stage17): merge_rate alone outperforms every model that includes hub_score across all repo size tiers:

Tier	mr_only	mr+hub	Delta
All medium+	0.516	0.408	-0.108
Large (500-1999 PRs)	0.553	0.484	-0.069
XL (2000+ PRs)	0.533	0.405	-0.128

log_account_age adds nothing (stage19): On 4 stable cutoffs (n=130 to n=1014, 5-fold CV), mr+age never beats mr_only. DeLong p > 0.07 at every cutoff. age_only AUC is 0.505-0.522 (barely above chance).

Cutoff	N	mr_only	mr+age	DeLong p
T_2022	130	0.584	0.576	0.807
T_2022-07	431	0.606	0.606	0.992
T_2023	474	0.552	0.534	0.076
T_2024	1014	0.580	0.569	0.111

Recency windows don't help (stage18): No significant difference between alltime, 2yr, 1yr, 6mo, or 3mo merge_rate for unknown contributors (zero significant DeLong tests across all tiers and cutoffs).

Cross-repo merge prediction confirms (stage13): hub_score hurts here too. ge_v2_proxy (hub_score + merge_rate) AUC 0.542 vs merge_rate_only AUC 0.576.

PR #27 validation study corroborates: account_age was LRT-significant (p = 1.2e-5) against graph_score but did not improve AUC ranking (DeLong p = 0.65 for GE + merge_rate + age vs GE alone).

Implementation tasks

• Remove hub_score (graph_score) from the scoring formula in scorer.py
• Remove log_account_age from the scoring formula in scorer.py
• Replace the v2 3-feature LR with alltime merge_rate as sole input for unknown contributors
• Keep graph construction (needed for repo discovery and contributor mapping)
• Keep skip_known_contributors logic (fast-tracks known contributors)
• Update thresholds to map merge_rate directly to trust levels
• Update docs and config reference

[jeffreyksmithjr]

Fresh Egg (account age warning) split out to #47 -- informational flag for accounts under 1 year, based on stage 20 changepoint analysis.