chore: migrate max_assignment_limit to agent capacity policies

Adds a data migration and rake task to convert inbox max_assignment_limit
settings into AgentCapacityPolicy records with per-inbox conversation
limits, preserving the exact capacity-based semantics.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/tasks/migrate_max_assignment_limit.rake

@@ -0,0 +1,11 @@
+# Manually run the max_assignment_limit to agent capacity policy migration.
+# This is idempotent and safe to run multiple times.
+#
+# Usage:
+#   bundle exec rake assignment:migrate_max_limit
+namespace :assignment do
+  desc 'Migrate max_assignment_limit from inbox config to agent capacity policies'
+  task migrate_max_limit: :environment do
+    MigrateMaxAssignmentLimitToPolicies.new.up
+  end
+end