Fix duplicate new contributors appearing across multiple months

[castrojo]

Summary

• Fixes critical bug where contributors appeared as "new" in multiple consecutive months
• Root cause: Date boundary overlap in historical contributor query
• Solution: Subtract 1ms from historical query endpoint to ensure strict separation

Problem Details

Contributors were incorrectly marked as "New Lights" (first-time contributors) in multiple months:

Example duplicates found:

• KiKaraage: Marked as new in both Dec 2025 and Jan 2026
• coxde: Marked as new in both Dec 2025 and Jan 2026
• sebjag: Marked as new in both Dec 2025 and Jan 2026
• theMimolet: Marked as new in both Dec 2025 and Jan 2026
• jumpyvi: Marked as new in both Dec 2025 and Jan 2026

Root Cause

Date boundary overlap in scripts/lib/contributor-tracker.mjs:

Before:

// Historical query: 2024-01-01 to reportStartDate (inclusive <=)
// Current report:  reportStartDate to reportEndDate (inclusive >=)
// Problem: reportStartDate included in BOTH ranges!

PRs merged at exactly 2026-01-01 00:00:00.000Z were counted in both the historical set AND the current report period.

Solution

Subtract 1 millisecond from historical query endpoint:

const historicalEnd = new Date(beforeDate.getTime() - 1);
// Historical: 2024-01-01 to 2025-12-31T23:59:59.999Z (excludes Jan 1)
// Current:    2026-01-01T00:00:00.000Z to 2026-01-31T23:59:59.999Z
// Result: No overlap!

Verification

Ran comprehensive verification showing:

✅ Zero duplicates between December and January new contributors
✅ All 16 December new contributors correctly categorized in January:

• 7 active as returning "Wayfinders" (without gold highlight)
• 9 inactive (not listed)

Verified examples:

• ✅ KiKaraage: Dec (new) → Jan (returning, no highlight)
• ✅ coxde: Dec (new) → Jan (returning, no highlight)
• ✅ sebjag: Dec (new) → Jan (returning, no highlight)
• ✅ theMimolet: Dec (new) → Jan (returning, no highlight)
• ✅ jumpyvi: Dec (new) → Jan (returning, no highlight)

Changes

• scripts/lib/contributor-tracker.mjs: Fixed date boundary logic with 1ms subtraction
• reports/2025-12-31-report.mdx: Regenerated with corrected logic
• reports/2026-01-31-report.mdx: Regenerated with corrected logic

Impact

• Future reports: Will automatically use corrected logic (no code changes needed)
• Historical reports: Can be regenerated with --month=YYYY-MM flag to apply fix
• No breaking changes: Report format and structure remain identical

Testing

# Regenerate reports to verify fix
export GITHUB_TOKEN=$(gh auth token)
node scripts/generate-report.mjs --month=2025-12
node scripts/generate-report.mjs --month=2026-01

# Verify no duplicates
grep "highlight={true}" reports/2025-12-31-report.mdx | wc -l  # 16 new
grep "highlight={true}" reports/2026-01-31-report.mdx | wc -l  # 10 new (all different)