feat: add bd slot commands for agent bead slot management (gt-h5sza)

Add slot management commands:
- bd slot set <agent> <slot> <bead> - set slot (error if occupied)
- bd slot clear <agent> <slot> - clear slot
- bd slot show <agent> - show all slots

These enforce cardinality constraints for agent bead slots.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/migrations.go

@@ -46,6 +46,7 @@ var migrationsList = []Migration{
 	{"gate_columns", migrations.MigrateGateColumns},
 	{"tombstone_closed_at", migrations.MigrateTombstoneClosedAt},
 	{"created_by_column", migrations.MigrateCreatedByColumn},
+	{"agent_fields", migrations.MigrateAgentFields},
 }
 
 // MigrationInfo contains metadata about a migration for inspection