refactor: remove JSONL fork protection (dead code after Dolt migration)

Delete fork_protection.go and its test — these only protected
.beads/issues.jsonl from being committed by forks, which is no
longer relevant with Dolt-native storage. Remove ensureForkProtection()
call from main.go PersistentPreRun.

Part of bd-9ni (JSONL removal Phase 2+3).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Executed-By: beads/crew/emma
Rig: beads
Role: crew

Author: steveyegge