feat(contributor-growth): recent-activity check and stricter pre-filter options for candidate-screen

[potiuk]

Summary

candidate-screen's pre-filter kept anyone whose merged PRs or reviews
reached half of a floor over the whole window. On a large project that
keeps people who stopped contributing months ago, and far more people
than anyone reads; the ratio was the only knob.

Three settings in contributor-nomination-config.md control it now:

Key	Default	Effect
screen_recent_activity_months	4	Drop anyone with no change landed in the last N months of the window, for both targets; 0 turns it off
screen_prefilter_require	any (previous behaviour)	all requires both merged PRs and reviews to reach their share of the floor
screen_prefilter_targets	committer (previous behaviour)	both applies the count check to the governance-body pool too

The recent-activity check is one more count-only query per person
(list_authored(state: landed, since: end - N months, count_only)).
Every drop is still logged with its counts and the check that dropped it,
and the floors are still used only here.

The new default for screen_recent_activity_months changes behaviour
for existing users: people with nothing landed in the last four months
are no longer measured. Set it to 0 to keep the old behaviour.

Type of change

• Behaviour change (skill)

Test plan

• New pre-filter eval case: a contributor with no recent landed change
  is dropped, require: all drops one who reaches only one floor, and
  targets: both drops a governance-body candidate below both floors
• Existing pre-filter cases pinned to screen_recent_activity_months: 0
  so they keep testing what they tested
• prek run --all-files passes

Generated-by: Claude Opus 5

🤖 Generated with Claude Code