feat: add type: event for operational state changes (bd-ecmd)

Adds support for event beads that capture operational state transitions
as immutable records. Events are a new issue type with fields:
- event_kind: namespaced category (patrol.muted, agent.started)
- actor: entity URI who caused the event
- target: entity URI or bead ID affected
- payload: event-specific JSON data

This enables:
- bd activity --follow showing events
- bd list --type=event --target=agent:deacon
- Full audit trail for operational state
- HOP-compatible transaction records

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

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

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

internal/storage/sqlite/migrations.go

@@ -49,6 +49,7 @@ var migrationsList = []Migration{
 	{"agent_fields", migrations.MigrateAgentFields},
 	{"mol_type_column", migrations.MigrateMolTypeColumn},
 	{"hooked_status_migration", migrations.MigrateHookedStatus},
+	{"event_fields", migrations.MigrateEventFields},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -104,6 +105,8 @@ func getMigrationDescription(name string) string {
 		"created_by_column":            "Adds created_by column to track issue creator",
 		"agent_fields":                 "Adds agent identity fields (hook_bead, role_bead, agent_state, etc.) for agent-as-bead pattern",
 		"mol_type_column":              "Adds mol_type column for molecule type classification (swarm/patrol/work)",
+		"hooked_status_migration":      "Migrates blocked hooked issues to in_progress status",
+		"event_fields":                 "Adds event fields (event_kind, actor, target, payload) for operational state change beads",
 	}
 
 	if desc, ok := descriptions[name]; ok {