Add Jetstream config for optimize-multi-actiontail-fox-modals-for-existing-users-v2

[rfambro]

Adds a custom Jetstream config for the optimize-multi-actiontail-fox-modals-for-existing-users-v2 experiment.

What this adds

Metrics (weekly + overall):

• is_pinned — built-in but not auto-applied; listed so Jetstream surfaces pin rate per branch (primary fix; Experimenter UI currently shows no pin data)
• imported_bookmarks, imported_history, imported_logins — built-in binomials for browser-import rate
• main_crashes, content_crashes, startup_crashes, shutdown_hangs — built-in guardrails

Default-browser rate, retention, DAU, search, and ad metrics are omitted because they auto-apply.

Segments (6 custom lapse buckets based on last DAU before enrollment):

• last_dau_28_plus_days_ago (superset catchall)
• last_dau_28_to_34_days_ago
• last_dau_35_to_59_days_ago
• last_dau_60_to_119_days_ago
• last_dau_120_to_179_days_ago
• new_or_last_dau_180plus_days_ago

All six use active_users_last_seen (defined under [segments.data_sources]), modeled on os-notification-split-view-release-lapsed-users.toml.

Experiment context

• Nimbus: https://experimenter.services.mozilla.com/nimbus/optimize-multi-actiontail-fox-modals-for-existing-users-v2/summary/
• Brief: Experiment: Optimize multi-action/tail fox modals for existing users
• Target: existing users (profile age 7+ days) who are not default and/or not pinned, shown a startup Window Spotlight message
• Feature: fxms-message-7
• Dates: 2026-03-23 to 2026-04-20 (7-day enrollment)

🤖 Generated via /create-custom-config