Cleanup: dated sunset for private .spl export readers

[pewpewpotato]

Problem

Private .spl readers remain for context export / migrate-once. Without a dated sunset we keep half a VCS forever.

Who it’s for

Maintainers; teams still mid-migrate.

Acceptance criteria

• Documented target date or release for removing private .spl export readers
• Criteria for “safe to remove” (e.g. no supported dual-path; migrate-once declared end-of-life)
• Plan for what happens to context export after sunset (remove command vs fail with migrate-done message)
• No expansion of internal/repository / related for non-export features before sunset

Priority

Third of the post-pivot cleanup slice.

Owner

Spool Product Manager (date/policy) + Architect/TL (tech)