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/033_event_fields.go

@@ -0,0 +1,50 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateEventFields adds event-specific fields to the issues table.
+// These fields support type: event beads for operational state changes.
+// Fields:
+//   - event_kind: namespaced event type (e.g., patrol.muted, agent.started)
+//   - actor: entity URI who caused this event
+//   - target: entity URI or bead ID affected
+//   - payload: event-specific JSON data
+func MigrateEventFields(db *sql.DB) error {
+	columns := []struct {
+		name string
+		def  string
+	}{
+		{"event_kind", "TEXT DEFAULT ''"},
+		{"actor", "TEXT DEFAULT ''"},
+		{"target", "TEXT DEFAULT ''"},
+		{"payload", "TEXT DEFAULT ''"},
+	}
+
+	for _, col := range columns {
+		// Check if column already exists
+		var columnExists bool
+		err := db.QueryRow(`
+			SELECT COUNT(*) > 0
+			FROM pragma_table_info('issues')
+			WHERE name = ?
+		`, col.name).Scan(&columnExists)
+		if err != nil {
+			return fmt.Errorf("failed to check %s column: %w", col.name, err)
+		}
+
+		if columnExists {
+			continue
+		}
+
+		// Add the column
+		_, err = db.Exec(fmt.Sprintf(`ALTER TABLE issues ADD COLUMN %s %s`, col.name, col.def))
+		if err != nil {
+			return fmt.Errorf("failed to add %s column: %w", col.name, err)
+		}
+	}
+
+	return nil
+}