Fresh Egg: flag accounts under 1 year old in scoring output

[jeffreyksmithjr]

Summary

Add a "Fresh Egg" section to the Good Egg scoring output that flags accounts younger than 1 year. This is a simple informational warning -- it doesn't affect the trust score, just surfaces the data point for reviewers.

Background

Stage 20 analysis (experiments/bot_detection/stages/stage20_fresh_egg.py) found that:

• The v2 new_account_days = 30 threshold is too conservative -- almost no OSS contributors are that new
• Piecewise changepoint regression finds a merge-rate transition at ~3.9 years (median across 3 cutoffs)
• A 1-year threshold (near p2 of the age distribution) catches genuinely new accounts without over-flagging
• Accounts under 1 year have merge rates ~16pp lower than the population average

Account age doesn't improve merge prediction (#46 evidence), so it stays out of the scoring formula. But a young account is still a useful data point for human reviewers.

Behavior

• If account age >= 365 days (or unknown): no output at all -- don't add noise
• If account age < 365 days: add a "Fresh Account" section to the output with:
  • Account age in days
  • Account creation date
  • A brief note that this is informational only

This applies to all output surfaces: GitHub Action check-run comment, CLI, JSON, and MCP.

Implementation

• Add account_age_days and created_at to the TrustScore or UserProfile model (whichever is more appropriate) if not already present
• Add a fresh_account section to the formatter output, gated on < 365 days
• The GitHub Action comment should include a small section like:

  ### Fresh Account
  This account is **42 days old** (created 2026-01-25). This doesn't affect the trust score but may be relevant context for reviewers.
  

• No config needed -- 365 days is the threshold, period

Related

• Part of v3 redesign: Good Egg v3: simplify scoring to merge_rate only for unknown contributors #46
• Data analysis: stage 20 on bot-detection branch (results)